Got my PS Vita yesterday. Wow, it is one really nice piece of hardware. The interface is pretty good. A few rough edges, but if it's anything like the PS3 it'll get a lot of improvements in the next year.
It feels a bit like an early version of iOS to me. text entry, for example. it's lacking any way to select text and copy/paste
web browser is pretty awful. They should just use webkit like everyone else. The Google maps app is also terrible :-(
The download/notification list is quirky and could use some improvement.
Background music is awful, but at least the music in the launcher can be turned off (via a checkbox buried in the settings app).
Everything else works really well, though. So far I've just been playing demos. Haven't bought any real games yet. Looking forward to being able to transfer the few PSP games I have (which aren't compatible yet) and all the random PS1 games I've gotten from PS+. (Here's hoping Vita games appear in PS+ at some point, too. Seems like it would be a good idea to get some discounts posted soon, at least.)