I'm not going to try to spoil this for anyone, but I'm not going to make much effort to avoid spoilers, either, so read at your own risk.
Overall Impression: Worth seeing. It definitely beats X-Men III: The Last Stand, but I don't think it beats X-Men or X2: X-Men United.
It's hard to have an opinion on how this ended, but to go into that now would be jumping the gun a bit. For starters, let me say that this is another origin story. It goes back as far as the 1940's, where we see some formative events in the lives of Professor X, Mystique, and Magneto. The scenes around Magneto mirror and then expand upon what we learned about him in the first X-Men movie.
Let me take a moment here to note that whoever wrote the screenplay for X-Men: First Class has a serious crush on Magneto.
I'm a little disappointed with the character choices for the villains in this movie. The Hellfire Club is a pretty well established villain group in X-Men comics (from my 80's and 90's based recollections), and I don't think that this movie has done it justice. Yes, Shaw is a decent Big Bad, but I don't think he resembles his comics incarnation very much in either powers or motivation. Emma Frost is practically a Big Bad in her own right, so she shouldn't be playing second fiddle so much, and what's with the "turning to diamond" thing? (Update: Apparently that's an additional power she picked up in the comics sometime after I stopped reading regularly.) The henchmen seem to be clones of Nightcrawler and Storm, and I have no idea where the idea for the dragonfly lady originated.
Unsurprisingly, the "first class" X-Men line-up doesn't much resemble the original Marvel comics line-up. They seem to be trying to have a movie continuity that's separate from comics continuity, so the absence of Ice Man is understandable, but about the only X-Man recognizable from early comics is Banshee, and I'm not sure how they plan to resolve the little issue of Havoc and Cyclops being twin brothers (in the comics) when they've apparently made Havoc a generation older in the movies (Correction: They're apparently not twins; Cyclops is older.).
So, not much attempt at direct continuity with the comics. Fair enough.
Was it necessary to make Professor X a naive ignoramous? To the point of being an arrogant ass? By the end, it's kind of hard to sympathize with him or his ideology. He wants mutants to be able to integrate peacefully with the rest of humanity, but his own research (for his PhD dissertation) claims that such a peace is impossible. And the human governments of the movie bear out his research by being paranoid ass-hats who try to kill the people who just saved their butts.
So we're left pretty completely sympathizing with Magneto's position: kill the humans before they kill the mutants. Would it have been that hard to have at least a few human voices on the mutant side at the end?
Kind of a broken message, if you ask me.