I've been playing it the last few days. I'd definitely give it $20 if you enjoy Assassin's Creed games.
The graphics are nothing to write home about but considering how big the world is and that it is on a handheld, they're not terrible. The gameplay is more or less the same as any other AC game but with a couple new things here are there (most notable is the different personas. You can switch between an assassin's outfit, a slave outfit and a lady outfit in order to have special perks for missions).
AC3 Liberation is definitely worth $20. It's a lot of fun as long as you are not expecting too much going in. I have been pleasantly surprised based on the mediocre reviews.
Also, the whole persona outfits is a little unique, but does not sell the game for me. What sells it for me is that it is a perfectly good (if not very good) open world type game on a handheld. I'm hoping it lays the groundwork for a future Vita AC title that's even better.