I have two that I purchased pretty early. Unfortunately the phone dialer app only has basic functionality, and because of that I can't recommend them. Also, there is no provisioning support outside of using the Unifi Controller software. The software is pretty good, and I am using it for managing WAPs and switches. I just wish they would open up or document provisioning from and HTTP source or something. I have also had problems with touch screens on the phones, they are both off in relation to where they register touch, and there is no way to adjust the units that I have. I do have very early units, so this may have been resolved.
I have had much better luck with the Grandstream (apparently Grand-stream is a bad word) GXV-3275, which is also Android based. These phones are not perfect either, but we have been working through the issues, and the manufacturer has good support available through their site, and frequently updates phone firmware. I have about 130 of them deployed.