OK, well, after looking him up on IMDB, I realize that I may have seen him in Apollo 13, but I'm pretty sure that movie doesn't count because
A/ I only saw about half of it on a rainy day in sixth grade when we had a substitute teacher
B/ Tom Hanks is in that movie, and I dislike him so heartily that I probably spent the whole movie reading a book instead of actually watching it.
Where was I? Oh yes, X-Men: First Class. It's an entertaining film. It's kinda cool to see Xavier so smarmy and young, a bit of a party animal, instead of the wise, middle-aged father figure he becomes in later years. As always, he fails to be as interesting as Magneto. Magneto's darker, more passionate, and far more sympathetic. Plus, his powers are about a hundred times better than boring old telepathy. Michael Fassbender did a great job with him, bringing his own take to the character while still making it seem plausible that this Erik could grown up into the one we see in the other X-Men movies.
That said, I don't think this quite gels with the other X-Men movies. Is it meant as a reboot of the series or a true prequel? If it's a prequel, I'm pretty sure there's continuity errors, although to be honest I don't remember the trilogy to name anything specific. There's definitely some tribute to the other series, with cameos from Hugh Jackman/Wolverine and the actress who played Mystique in the older films. (I wouldn't have caught that one if Jeannie hadn't pointed it out.)
Special effects are laughably bad. Emma Frost/White Queen looks pathetic in her 'diamond' form. I mean, I've seen video game characters from the mid-2000s that look more believable. Ignoring everything else, there's a noticable drop in quality within the franchise. Mystique's costume in the very first movie, back in 2000, was pretty neat-looking. It had this cool scaly effect. But this version of Mystique looked like they'd just dumped the actress in whatever the Blue Man Group uses to achieve their brilliant blue hue. Very disappointing!