UPDATE:
After about two years, the green LED on one of the keys stopped working (basically, it was only red and blue). I got the exact same keyboard through the warranty, excellent service from the store. The second keyboard, for some time now, has been displaying the same letter twice when I press the button once. I have cleaned it and it continues to do so, so it may be a circuit board issue.

I bought it because my other peripherals are Logitech and I wanted uniformity (I had the Razer Blackwidow X Chroma) and because Logitech products, in my opinion, are top-notch.
Pros:
-The GX clicky switches have a very nice feel to the click without making a loud noise when they "hit" the bottom of the keyboard (compared to the Razer I had).
-Extremely compact, frees up a lot of space on the desk.
-Very nice feel to the touch of the keys, they are matte and the finger does not easily slip.
-The LEDs are very satisfactory, without bleed around and without being too intense.
-The G Hub, although a bit strange at first, is ultimately incredibly fast and unbeatable when it comes to changing profiles between games.

Cons:
-You can only assign macros or map to the F keys and not to each individual key.
-In the lighting profile, if you want something more complex, you need to spend some time to make it to your liking. If any of the default effects it has suits you, it doesn't create a problem.
-No keycap puller is provided in the package for easy removal of keycaps for cleaning.

It is an excellent keyboard and I recommend it to someone who spends hours writing and playing

Well, the keyboard is the best thing I've had for a month, everything is great, not to say much. There are a few things that bothered me a little, so I gave it 4 stars. However, I still recommend it, it's just that the price is a bit expensive for what it offers. I bought it for 130 euros, I think 100 euros would have been a fair price. Also, the program doesn't have enough color combinations as I wanted, compared to other keyboards. Nevertheless, Logitech for the win, don't hesitate to get it if you have the money.

Edit 12/8/2019
After a few months, I don't remember when I bought it, the Windows key had a different color than it should. When I press red, it shows green. When I press green, it shows blue, and so on. All the other keys in terms of colors and functions are fine for now. It's just that this issue is visually unappealing.

Edit 8/10/2019
The B key had the same issue as the Windows key, a different color from the other keys and the Windows key. It's probably a hardware problem. Anyway, it's a minor issue and it's still colorful.

Edit 5/11/2019
Today I noticed that another key has the same color issue as the F5 key. In total, there are 3 keys with this problem: the Windows key, the B key, and the F5 key. I'm starting to get disappointed.

Edit 7/11/2019
Lol guys, now another 4 keys (total of 7) have stopped working properly and each key has a different color. I'm really disappointed, but at least they still function as keys and I can still type. I won't mention the color issue again, 7 keys are enough. Only if some keyboard function stops working, then it's a real disappointment

I bought the keyboard for 70 euros on Black Friday, and for the usual price of 150 or 130 euros, it's not worth it at all. However, if you have money to spare, it is definitely one of the best keyboards by far.

The keys are very good in terms of construction and feel. They are softer than the brown buttons I had before, but they do make some noise. I would say they are also slightly quieter than the brown ones, but the difference is not significant.
[21/1/19] After two weeks of use, I realized that every time I press a button, there is a metallic spring sound coming from the bottom, and it's the same for every button. It's particularly annoying, and while the keys are nice, this is unacceptable. The sound gets louder the harder you press, and it's noisy! This seems to be some kind of issue with the Romer-G switches rather than a problem with the keyboard itself, as I read on a Logitech forum that others have experienced it too. So, there is a reason why Cherry MX switches are the standard in the market.

The colors on the keyboard are the best, truly incredible and by far the best on the market, while the Logitech software is not the best for many reasons.
[21/1/19] Also, I should mention that there is no brightness adjustment, it's either too bright or off.

In terms of construction, it's incredible, and in terms of size, it's the best by far. You only need the numpad if you work with spreadsheets and Excel or if you work at a cash register, otherwise, it just takes up valuable space.

My biggest issue is the fact that you can't change the FN switch to activate the Function keys instead of the default ones, which is really bad because you can't increase the volume by pressing a single button, and you have to press FN + pause, which is very annoying. The other option is to change some Function buttons, but then you lose those buttons.

This size is the best for me by far, the keys are very good (although there is no significant difference from the browns) in terms of feel, but the noise of the springs is so annoying that you can't bear it without wearing headphones. The Logitech software is mediocre at best.
Also, it's great that the cable is detachable, so you can move it without damaging it, or if it breaks, you can replace it. It's very fast, but you don't even see it, those are just marketing claims. It's incredibly sturdy and heavy, and its construction is amazing.

Since I stopped paying attention to the noise the Logitech buttons make, I don't recommend it to anyone. It's a wonderful keyboard that no one should buy.

[9/9/2019]
For some unknown reason, Logitech has two software for their gaming peripherals, the Logitech Gaming Software and the Logitech G Hub. The LGS is the old one, which is very slow, doesn't do what it says, and is generally incredibly difficult to use and outdated. On the other hand, the LGH, which was released in 12/18, is an incredible program that fulfills every standard one can have, and now there is no issue with the FN key, which I'm very happy to say works with the LGH unlike the LGS.

A friend of mine got the new one and now it has GX Blue keys instead of Romer G, and both are from Logitech but these are expensive copies of MX Cherry. While in general I believe they are better, their lighting is very inferior as now there is only one LED on top that illuminates only the upper button with minimal intensity, while the lower indicator for play/pause, etc. doesn't light up at all.

I will start with the basics: I am peculiar!
The main use of the keyboard is for typing texts.

I will begin with the negatives:

1. The keyboard has a metal plate at its base. This makes it heavy and sturdy. However, when you hit the keys and they reach their destination, in a quiet environment, you can hear a metallic noise coming from the plate, as if you were hitting a pot, which gradually fades away. Particularly annoying in tone.

2. The keys do not change color when pressed (except in a useless mode). There is no option.
You cannot, for example, use the Caps Lock key to set the key itself or any other key to change color. For me, this is a tremendous omission.
I want the keys to change color when I hold down the Shift key. It would be so convenient, but it DOES NOT happen!
I want the arrow keys to blink, while the rest have a steady color. It DOES NOT happen.
You press, for example, the accent key (";") and only the vowels light up. It DOES NOT happen.
It has one tone effect, but you cannot create something practical and substantial. A great negligence.

I emphasize: The color change must (is necessary) be possible with each key press and not just as a profile.

3. You cannot change the color of the keys when you are in a profile. For example, in the Word profile, when I press a combination of keys, the color of the keys should change... IT DOES NOT happen.
However, you can achieve it with tricks. You create 2 profiles for the same program and with a combination of keys, you cyclically change the profiles. Detailed instructions here:
https://www.youtube.com/watch?v=64rzuV7bjY8
I did it for Word and the letters have a different color in Greek and English (sometimes, due to speed, it loses the profile change). However, it would be great if it could be done in the same profile.
I hope for an upgrade.

4. The keys have slightly thick characters.

5. You cannot adjust the brightness of the backlight. The backlight is On / Off. However, it has a separate switch for this (top right corner).

6. The keys do not have Greek characters (maybe it's better that way).

7. The famous G-keys are only the F-keys, that is, F1 to F12.
Only in these keys can you assign macros and combinations.
Unacceptable!

8. Very expensive. I got it on sale for 100 € and I'm still thinking about it...

Positives:

1. It is a jewel! Beautiful with a matte finish. The plastic has a perfect finish.

2. The keys have strong backlighting. (Personally, I chose a color with reduced brightness because it tires me. I didn't choose green, but dark green, because as I mentioned above, you cannot reduce the brightness.)

3. The backlight does not spread between the keys.
Tremendous advantage!
You only focus on the letters.
(For some, this may be a disadvantage.)

4. The keys are soft, with a short travel distance.
They are almost silent with a plastic tone. That's why you can hear the unacceptable metallic noise coming from the base.
(I am thinking of opening it and adding pads so that the metal plate comes into contact with the plastic cover and reduces vibration.)

5. It is recommended for typing and is very pleasant.
It can only be compared to a very good laptop keyboard. Those that have a short travel distance and are very soft to the touch.
The removal of the numeric keypad is a great advantage because the mouse is close to the keys and the mouse-keyboard movement does not tire. I still wonder how they write with long keyboards and the mouse sitting on the other end of the desk. Besides, there are separate numeric keyboards for every possibility.

6. Each profile can run a script, which gives it great flexibility. However, this can only be done for the G-keys, that is, F1 to F12. It is unacceptable that you cannot assign any key as a G-key.

7. Heavy and sturdy, with non-slip pads and adjustable feet, with 2 options.

8. It has a separate switch and you can disable some keys from accidental presses. For example, you can disable the Win key and any other key you want when playing games.

So, is it worth it?
For €100, I am borderline saying NO.

The "frills and scents" with the colors are nice, but the essence of the matter is small and ultimately has limited capabilities.

It could, when inactive for a few minutes, automatically switch to one of the impressive effects it has. And as soon as you touch it, it returns to the profile it was on.

The "right" price for it is €80.
It would be worth €100 and more IF it could do what I describe in the negatives.