Ian McKellen chews up the scenery wonderfully in this one. The major plotline revolving around him, Xavier and his school, and the mutant politics is well done, and we do get to see what Magneto can REALLY do. Up until now, he's just been playing.
The Phoenix storyline, unfortunately, was botched.
They really should have left this to be its own movie -- either done Phoenix, or done the Mutant Problem storyline, but not both. And at the points where they could at least have done it some justice, they fumbled the ball. I'm sorry, but it's not Phoenix without the firebird. It's not Phoenix without "HEAR ME, X-MEN! NO LONGER AM I THE WOMAN YOU KNEW! I AM FIRE, AND LIFE INCARNATE!"
They really needed to do that as two or three separate movies: one for the Return of Jean Grey as Phoenix (which COULD have worked with this one, ending with an all out power slugfest between Jean and Magneto), then the Hellfire Club/Dark Phoenix.
There were a lot of good points in the movie, however. Kitty Pryde was DEAD on, as was Colossus (and yes, we even get the Fastball Special!). Kitty beats Juggernaut... with her brain, as well she should. We get to finally see The Iceman for real. Excellent combat scenes. Wish Storm had been allowed to go all-out, though.
This one is worth watching in the theater for certain scenes that just aren't going to be as impressive on the small screen, but if you're not really into the X-men I'd wait for the DVD.
The new trailer for Superman Returns looks dark but promising. And they've done Ghost Rider... and Johnny Blaze looks pretty darn cool.