I use it in a small hotel as a central router and controller. I was looking for something that had the capability for DNS filtering (via the NextDNS CLI) and could run some scripts automatically at every startup—even when I am not present.
I was surprised that it remained absolutely stable and functional even in cases of low voltage. The small built-in display is ok, somewhat useful, mainly for quick checks (uplink, firmware updates, etc.). I have mounted it on a wall (with the inline 19" 1U rack for under-desk mounting) without any problem—it's relatively lightweight.

Pros:
− Stable and reliable under any condition
− The built-in firewall, relatively flexible and with quite a few features
− Silent
Cons:
− There is no way to exclude specific IPs from the captive portal

It is expensive but it does a real job, I have had it for about 20 days and with the rest of Ubiquiti products, AP, Switch, camera, etc. it has flawless collaboration in a unified control panel.

When I first bought it, I would say I was a beta tester… its operating system was in a very infant stage and had many bugs. Two years later and after many updates, it is a very capable router/recorder/firewall. I use it with a switch, access points, and cameras, all UniFi, and I am very satisfied.

Update 11/8/23
Every update gets better, I use it with two WAN in failover mode and I have had zero downtime for a while now.

Update 20/3/25
Every update gets better. Ad blocking and HA (I don't use it yet) are some of the upgrades it has received in recent years. Investment!

What to say first about this machine.
For my 100Mbps connection, it's completely overkill, but it has so much processing power and capabilities that I simply couldn't resist.
It functions as a firewall, router, switch, uisp controller, and as an NVR for Unifi cameras.
Its touch screen has helped me quite a bit, more than one would expect.
The initial setup takes about 10 minutes, and if you have the mobile app, even less, but it includes basic things... of course, you can do WHATEVER you want, but it requires deep knowledge of EdgeOS/Linux if you don't have the help of Google.
It's the only Ubiquiti device I have, so I can't evaluate the capabilities of its controller.
I've installed 2 Dell 10Gbps transceivers in the SFP ports and it recognized them flawlessly, providing full speed to my servers with open QoS.
That's all from me... if you have the money and enjoy dealing with networking, it's the best thing you can get to experiment and gain practical knowledge in the IT field.

UPDATE: With 4 cheap gigabit media converters, I sell internet with fiber optics to neighbors who don't want to pay for a full package for a simple internet connection... through the UDM, I assigned a different VLAN to each port so that the subnets don't have contact with each other, and through firewall rules, I set speed limits for each one... I also provide access to my Plex server, which is located in my personal network... essentially, I pay almost nothing to share a few Mbps occasionally, which I don't even notice in whatever I do online.

Excellent machine, worth its money. Some relevant knowledge or familiarity with networks and their peculiarities is necessary, it is not completely plug and play but nothing out of the ordinary. Highly recommended.