I really enjoyed this film. It's easily beats out X3 and Wolverine. McAvoy, Fassbender, and Bacon were all great and were given good material to work with. The rest of the cast wasn't bad either, though most of the X-Men didn't get much to work with, besides Mystique and Beast.
I thought the cameos were fun, the FX were good, the story well paced and I liked how they tied it to the Cuban Missile Crisis.
I didn't like that they killed off Darwin. Also didn't think that January Jones was sexy enough to be Emma Frost. She's pretty, but she doesn't radiate lust. Though Jones handled her part well enough. I was thrown off, but pleasantly surprised by the Professor X-Mystique relationship. I also like how they developed Mystique's journey, making her a pretty sympathetic character. I also like how they handled Beast, his relationship with Raven and his own self-loathing and eventual acceptance of his mutation.
I wished more time could've been given to Angel Salvadore, Havok, Banshee, and Darwin, if nothing else to flesh out their personalities a little more. Though I know the film was about Charles and Erik and this Vaughn pulled off pretty well. I also wish there had been a reference to MLK or Malcolm X, like in the first X-film, since the events of First Class were taking place during the modern Civil Rights movement, a movement that influenced the creation of the X-Men comic.