I think the opening box office numbers for X-Men are just fine. X3 and Wolverine Origins left a bad taste in people's mouths. There was no demand for another X-Men movie, especially with films like The Dark Knight and all the new Marvel (and DC) comics coming out. I didn't expect it to do the business it did in previous films. I think $50-$60 million is just about right for First Class, despite what Box Office Mojo thought in an article where they completely overreacted to the box office total.
The real test for the movie is to see what they do in their second week. The word-of-mouth has been very good and if they manage to retain more than half their audience, I say the film can be classified as a hit.