This a great addition to the franchise. I thought the film did a good job exploring the relationship between Charles (Prof X) and Erik (Magneto). And Kevin Bacon was a great choice as the heavy.
If you're one of those people (like me) who likes to have everything fit together neatly, don't bother -- they don't. Some of the events in First Class don't quite mesh with the earlier X-Men movies. Also, the humans vs. mutants conflict is a bit oversimplified; but these are minor quibbles. If you're an X-Men or comic book fan looking for guilt-free entertainment, check it out.