I've only had the Vita for a couple hours and most of my opinions on the system in general are positive with very minor quirks. So far my best and worst things about the system are:
Best: The UI is super fast and snappy. I can sync trophies, go to my friend list, send message and go back to a full game in a minute.
Worst: I don't much care for the back touch so far although I'm willing to be open as I have not tried every game yet. Also I should come to expect this from devices but my 16GB card only has 14 available, 2GB is kinda a lot to take away.