(Quick background, in case anybody doesn't know what this movie is about: this is the origin of the X-Men as a *concept*. It doesn't include most of the familiar characters like Cyclops or Jean Grey; rather, this is the background prequel that lays the ground for them.)
As previously mentioned, I don't expect comic book movies to be utterly faithful; rather, my metric is, "Did they *get* it?" In this case, I have to say that the answer is a smashing "yes". It doesn't quite match any existing rendition of the origins of the X-Men -- but frankly, is probably the best version I've seen to date. In particular, this is a complex and nuanced joint origin of Professor X and Magneto, showing the two of them creating themselves and each other in the early 1960s.
Of course, purists *are* going to get annoyed here and there. The timeline just plain doesn't work in some places. (They seem to have missed the fact that there are more than 12 years from 1944 to 1962.) They pulled several C-list characters into the 1960s, almost at random. And they kill the one character who it makes absolutely no sense to kill.
But those are quibbles. The important thing is, the main characters -- Charles Xavier and *especially* Erik Lehnsherr -- are played with real depth and complexity. Frankly, Magneto entirely walks off with the movie. Charles is a frivolous twit who turns into a self-righteous twit, but Erik is genuinely interesting and tortured, searching for a path. It's heartbreaking when he finally chooses his direction -- and unsettling that you can sort of see his point of view. There is no subtlety here, but his arc has the flavor of a Shakespearean tragedy: a man who has defined the boundaries of his life in a way that can only allow him evil as an outcome.
As for the considerable supporting cast, there are a bunch of interesting choices. Hank McCoy is played just right, the shy scientist slowly becoming the lovable blue Beast. Raven Darkholme actually gets an origin story that makes sense and ties deeply into the story for once; indeed, she gets more arc here than any of the simpler heroes do. And a few mid-level X-Men like Havoc and Banshee are pulled in and used to good effect.
On the villainous side, they violate much of what I know about Sebastian Shaw, but make him a much more chillingly evil character than his usual portrayal. As played by Kevin Bacon, he's a delightful 1960s Bond villain, frankly: stylish, over-the-top, and *quite* insane. His sidekick is, of course, Emma Frost -- a singularly shallow and annoying version of Emma, but that's kind of true to what she was like in the early days. (Plus a couple of random bad guys who I assume were minor X-Men villains who I don't even recognize, including one who can only be described as Dark Nightcrawler.)
Mind, it's still a Marvel movie: it's not going to win any Oscars. But they get pretty much everything right: story, script, cast, music, and of course special effects. This is an A-list action flick with enough brain to actually be interesting. A fine summer movie, worth seeing in the theater. (And as a special bonus, it's not in gratuitous 3D! Yay!)