I've been using UT since the Aquaris E4.5 came out. I really like the gesture based control, and whenever I have to use an android phone, it seems awkward and not intuitive.
I still have the E4.5 - my son uses it. I also got the MX4, which I'm using right now. Biggest problem is/was that it doesn't automatically reconnect when it loses coverage. This can be mitigated by waking it every 5 minutes (with cron), but otherwise it's a good phone. Battery life is not stellar though, and the cron job surely hasn't helped that either . The connection issue seems to be related to the baseband chip and not UT.
I also have a OPO, and it works pretty well. It doesn't lag the same way the MX4 does - even though it's slower. I can't use it as a daily driver though as bluetooth doesn't work, and that's a deal breaker for me, as I use 2-3 hours every day commuting.
I saw in another post, that this might get fixed soon, so I cross my fingers.
Anbox could be nice, but I only miss one android app (mobilepay - a danish money transfer app).
MMS doesn't work at all - and that's on all of the phones. This seems to depend on the operator, so I have no idea if it's ever gonna get fixed.