UB still leaves a bad taste in a lot of people's mouths after the scandal and then the Helmuth hand glitch.
That said, I have a couple bucks on there that I donk around with once in awhile when I'm taking a break from my regular grind. Just won a $2 SnG last night in fact. The competition is typically very soft because there's not a lot of top players there. Waiting around for SnG's to fill up can be excruciating, sometimes it's completely dead in there.
Their software SUCKS. It's probably the worst client software of any site IMO, and that includes Cake skins. They've tried to make it look nice, and the production values for their site and client overall are pretty high, but the actual gameplay experience is terrible. It's incredibly slow in some places (e.g. shoving pots to the winner), too fast in others (auto-fold processing), the audio is like nails on a chalkboard, etc. All minor nags that add up to a frustrating experience when you're used to PS or FT.
I wouldn't play them as my primary site. It's hard to beat PS or FT if you're a US player, so I'd tend to recommend them above all others. I'd probably go with
bodog or Carbon before I put a lot into UB/AP.