People who liked either the first movie or the New Moon book will probably have a great time at this movie. People who didn't like them still might have fun laughing at the moments of glorious teenaged awkwardness--Michael Welch returns as the hilariously down-on-his-luck Mike Newton.
As a sequel, New Moon is excellent. It maintains the same tone as the first movie, smooths out some of the rough edges and builds from there. On its own it's mostly quite enjoyable to watch. There were lots of intentional and unintentional humor and of course the movie takes only a bit of the edge off the book's more than slightly unbalanced story structure.
Things that impressed me? FIGHT SCENES. They actually let us see Felix toss Edward around like a rag doll. And they might have robbed us of the sight of spooky elder vampires petrifying in their old age, but they did let us see Edward's head literally crack like an eggshell as he gets slammed into the floor tiles.
I also like that they expanded Alice's role and showed Bella actually missing someone in addition to Edward. I really wish that both the books and movies had gotten more into the Bella-Alice friendship. Also, the idea that a person's spouse is the only relationship they need is a relatively new one. If even a romantically obsessed book series like Twilight can focus on the fact that Bella is reacting to the entire Cullen clan and not just Edward, then maybe we're moving toward a healthier idea of what love is.
Also, Michael Sheen is the champion of creepy. Sorry, Bill Nighy; he played Aro before you played Greyback! My final recommendation? Go see this movie with a big crowd for all the squealing and shouting at the screen. It's like a floor show but with body glitter.