Or… "I Now Pronounce You Chuck & Maggy".
For make no mistake, this film belongs to Professor X & Magneto. And really, that's at the heart of First Class's problems, It's not with the plot, really, which while a bit thin, is perfectly workable. And the effects are quite good, particularly when Magneto cuts loose.
It's with the characters, specifically the X-Men & "evil mutants". None of the proto-X-men (with the possible exception of Mystique) get anything remotely resembling a backstory, and very little personality. And that makes their various spotlight scenes feel rather thin. Without an emotional connection to a character, it's difficult to feel any reaction at their triumphs, betrayals, or even deaths. The character who dies seems to have been created to die, as he really did little else in the film, and even the way he died felt forced, if not an outright continuity error.
Shaw's minions are even worse, as one of them is never actually named, another is basically a palette-swapped Nightcrawler, and the third (Emma Frost, one of the more entertaining characters in the comics) comes across very flat. She could have used an infusion of dry wit, if not outright cattiness.
A word about Mystique: her character arc wandered all over the map, from sisterly concern, to a desire to conform, to a desire to rebel, all within the space of a couple of weeks. Looking at it from the outside, it felt like she never really had motivations of her own, instead latching on the nearest strong male presence and following his lead. The subtext is rather troubling.
For his part, Shaw comes across as your classic 1960's Bond villain, complete with undersea base and mirrored sanctum (what IS it with villains and mirrored rooms?). His motivations are so basic as to be almost ridiculous, based on a cartoonish idea of science that would have been easier to swallow if the rest of the film hadn't tried so hard to be plausible.
(I do have a theory that he could come back as Mister Sinister, though. They even provided a nifty explanation for his forehead diamond.)
That leaves the two main characters (and they ARE the main characters, no matter what the posters imply). Charles Xavier, as portrayed by James McAvoy, has the roots of peaceful mediator of the future, but the story makes him come across as a bit naive, particularly in the final confrontation with Shaw (during which he simultaneously screams at Erik to stop while not doing anything to stop him). Still, his gee-whiz charm (when he's sober, anyway) makes him an engaging character.
As for Michael Fassbender's Magneto. Wow. Even though his friendship with Xavier is a bit abrupt, he's by far the most interesting (and tragic) character in the film. I'd say that his performance, by itself, lifts the movie from merely average to quite good. Indeed, I could have easily watched a film about his hunt for fugitive Nazis, and had it end with him meeting Xavier for the first time. I vaguely recall hearing that this movie includes elements of the abortive X-Men Origins:Magneto project, which explains quite a lot.
And yes, there's a cameo (two, actually) tying it to the other films, but a picky fan could spot other cameos and plot points that indicate this can't be a prequel to those films. Doesn't make the movie any better or worse.
Is it a great film? Honestly, no. I've only hinted at some of the plot holes, but they are there, and the characterization shortcomings among the supporting cast are notable. But it is an interesting one, particularly when Fassbender is on the screen. And sometimes, that's enough.