"Not really."

Film Bleeding Cool asks "Who Really Wrote X-Men: First Class?". Never has a group of writers arguing in public over a screen credit for a script been quite so entertaining:

 Ben: There were other writers who had established parts of the script…

Matthew Vaughn: Not really.

Ben Mortimer: So it was you and Jane then?

Matthew Vaughn: The WGA don’t think that, but they’re fuckwits.

The primary source text on these things (as is so often the case) William Goldman's Adventures in the Screentrade which demonstrates over and over again that just because someone's name is on the screenplay, they didn't necessarily write it.  Goldman's written more scripts than you'd imagine and is credited for things he ultimately had nothing to do with.