Interesting approach, giving us an origin story as the fourth entry in a film series. But it kind of works here. We didn't need all this information for the first few films and by adding it at this point the experience is actually that much richer. I really liked the retro 1960s setting.
Good writing, directing, and acting all around. There are some very good scenes. It gets marked down a little because there are a bit too many scenes, perhaps because there are a few too many characters to cover with the story. The whole thing could have benefited by one more round of editing on both the script and the film. That would have probably taken out a couple of slower spots. And perhaps done away with the a couple of extraneous strikes to the head with the "us versus them" two-by-four. I think most people got the message well before the middle of the film. You don't have to keep going on about it.
All in all a fun flick for a rainy summer afternoon.
X-Men: First Class (IMDb)