Magneto was, in my mind, the only really compelling character in the 3rd film. I enjoyed the first and second X-Mens, but this last one left me cold. My understanding is that they changed directors. Whatever caused it, the drop in quality was marked.
They changed directors because Brian Singer was only doing X-Men in order to get superhero movies on his resume incase somebody decided to resurrect the Superman movies which was his dream directing job. And of course it worked because somebody did resurrect Superman and he got the job. I think the new director did a fine job, if there was a problem with the movie it's too many characters. In trying to get everybody's favorite they've had to spread the plot across too many characters, so you no longer have the time to get emotionally involved with the characters. It also becomes a festival of loose ends, where'd Nightcrawler go, how long ago was Beast an X-Man, did anybody actually bother to look for Cyclops and make sure he's dead? But the action was nice, the story was nice, and what you got to know about the characters was nice. It would have been better at a full 2 hours and with about 6 fewer characters.