A friend told me an interesting bit of trivia about the name of this film. Originally meant to be titled just plain X2 (as if X-Men 2 was too difficult to remember), studio executives at Fox requested it be retitled, for promotional purposes, as X-Men United because it's now passe to add a "2" onto your sequel's title. After all, it's not The Matrix 2 or Charlie's Angels 2, right? Ah, studio mentalities.
Anyway, I enjoyed X2 to the same degree I enjoyed the first X-Men. I liked it in general, had the same complaints (Storm's character is woefully underdeveloped, Cyclops is a complete ninny), but mostly just had a reasonably fun time wallowing in the nostalgia that both movies dredged up - that period of my life when, from the age of 9 to 12, I was a rabid fan of the X-Men comics. So once again I appreciate the attention to detail and the fine casting (the big improvement in X2 is the addition of Alan Cumming as the perfect Nightcrawler), though it's obvious that some of these superheroes' talents are cinematic, and some aren't: Poor Anna Paquin, so central to the first movie (as her character Rogue served to introduce the audience to the world of the X-Men as she was inducted into the group) doesn't have much to do now, since her mutant powers aren't visually that exciting.
However, I admit that I am enticed by the promise of the film's pseudo-cliffhanger that the sure-to-come sequel will center around my favorite chapter of the X-Men saga (aka the John Byrne years): The rise and fall of Phoenix. I'm sure my fellow X-dorks not only know what I'm talking about, but can also guess what that nebulous shape in the water is in the final shot of X2. Anyway, if you liked the first X-Men movie, you'll like this one too. If you didn't, then don't bother.