So far between Bleeding Cool and Collider, X-Men: First Class is getting fantastic reviews and reactions.
All the articles between those two sites seem to infer a few things about the picture as well.
1. It's a prequel, not a reboot, but it does alter the timeline in the culmination of one scene. Essentially rejecting the Xavier cameo in X-Men Origins, or to be explained later as to how this occurred. It also seemingly for now, rejects the flashback in X-Men 3 until further explanation later as to how it happened.
2. A special bar cameo scene fits snugly into the picture, makes sense, has one of the all time best lines in the movie, and still lets the prequel/sequels timelines stay intact.
3. This movie was made to have sequels if it does well. Vaughn has already commented on the next that he wants to add a certain unnamed mutant to the cast. (Quote withheld, because it has a major spoiler in it for the film.)
I'll be going more indepth about what fits continuity-wise and what doesn't after next Friday. That's the day I plan to go see it for myself.
From all the various review outlets though, it seems that Vaughn has hit this one out of the park.
X-Men: First Class hits theaters in the UK June 1st, and hits theaters in the US June 3rd.