Michael Fassbender of Inglorious Basterds fame played Erik Lehnsherr (a.k.a. Magneto) to the hilt but never as a caricature. His portrayal showed Magneto's pain and anger, and the menace that he can be because of all that. Oh boy, do I love to kill Sebastian Shaw as well in the most brutal way possible, but how he did it in the film was just perfectly brilliant (Oops!). Fassbender also has this commanding presence that fills every scene, easily drowning everyone else -- at least the weaker actors.
It doesn't hurt either that Erik was a fashionable guy in the film. The movie's costume designer, as I've read on GQ.com, took inspiration from the early James Bond movies as First Class was primarily set in 1962. And might I say that Fassbender does justice to those outfits very much.
Erik Lehnsherr has always been an interesting character to me, with a wealth of mystery buried within. Never mind that I seem to gravitate towards villains but even when he was a kid at that Nazi concentration camp, he was already awesome. That scene with Shaw? I loved it! (I also loved how they used the opening sequence from the first X-Men movie as again, an opening scene.)
In contrast, Charles Xavier (played by James McAvoy) was a privileged kid. I don't know but I've always found him boring. And it doesn't help that McAvoy has to constantly do the fingers-on-his-temple gesture through a lot of his scenes. But it's a big thank you to the Professor nonetheless for unlocking Magneto's true potential. I don't know if he's sorry at all for doing that.
In terms of beliefs, I side with Erik. Always have. He was right when he said that humans will turn against them after they've averted World War III. Charles refuses to see the truth, still firm on his belief that humans and mutants can coexist peacefully. And this, I think, is his greatest weakness.
As for the rest of the movie, the plot was convincing enough for me. It's always a challenge these days to make superhero movies more realistic, especially after The Dark Knight, but First Class pretty much delivers. With too many characters, however, character development was a problem for some of the supporting cast. But that's negligible to me anyway. As regards production and costume design, I sort of got the 60's feel there but not so much from the mutant kids they've recruited. They just look so now. The effects were good, a far cry from the first movie back in 2000.
Overall, X-Men: First Class is a movie I'd recommend watching. Now I can't wait for what's in store in the next installment.