So I saw First Class this Friday, and I liked the film a great deal. I will try and provide a review that does not give away much more than you have already seen in the trailers.
For those of you who have not seen the trailers (so for you this might be slightly spoiler-y) First Class tells the origins of Professor Charles Xavier and Magneto, former best friends who now stand at opposite ends of the fight for mutant kind. The tale takes place in 1962, and I think just about everything about the film was well done. The performances were great, the look and feel of the era was captured well (although I think some of the hair styles may not have been sixties accurate). A reviewer at IO9 compared the film to a James Bond flick, which made me smile when I read that because I did so after I saw the movie and during it my brother Donald said something along those lines when he compared Kevin Bacon to a James Bond villain. And that is a good thing, in my opinion.
If I have any complaint, it is that several of the proto X-Men do not get much opportunity to be fleshed out. And Banshee is nothing like the Sean Cassidy I liked in the comics; instead he is little more than a teenager. In watching the film I understand why that was done, but it was still a bummer not to get the classic Banshee, because likely we aren't going to get a classic Black Tom Cassidy as a bad guy (then again, we aren't going to get a classic Juggernaut, either, after that atrocious third X-Men film). And one bad guy does not get a single line of dialogue. Heck even Goldfinger's Odd Job got to grunt once in a while. :/
Another thing I thought was awesome was the origin of Magneto's helmet. If you have seen the trailers he is wearing the bucket. Where he gets it from is pretty neat, in my opinion.
I give the film a solid 8/10. If the younger X-Men had been given a bit more opportunity to shine I think the film would have been better for it, but I can understand with time constraints why this was not done. I am looking forward to the DVD release to catch the deleted scenes. I would love to go into more detail about the film but I do not want to give away the plot or discuss some of the more awesome aspects of the film. I highly recommend you see this one in the theater. With this film and Thor I think this Summer's movie season is much stronger than last year's.
Finally, do not bother staying to the end of the credits, there is nothing there like there had been in the last X-Men film or other Marvel movies. I just don't want you wasting your time.