This prequel, starring James McAvoy as Prof Xavier, definitely filled in some of the gaps the last four movies have kept secret. And not only that, but it did something that few prequels can claim, it had a relevant, interesting, multidimensional storyline.
Character development seemed to be a key aim of the film, with a significant portion devoted to the young lives and origins of each X Man/Woman. Their lives are even more intertwined than one would suspect from the previous four movies, with the exception, perhaps, of Erik- whom we all know to be Charles' best friend and arch nemesis (One of the most interesting relationships in sci-fi history, in my opinion).
James McAvoy plays Xavier (Professor X) to a T, capturing his brilliant mind, wit and supernatural calm, as well as his ability to bring people together and help them develop their abilities.
Michael Fassbender plays a haunted (And sexy) Erik Lehnsherr (Magneto), and you can't help but root for him as he goes about his killing sprees once you've seen his story.
Mystique/Raven is played by Jennifer Lawrence and her story is one that I haven't had the opportunity to explore via the comics. I don't want to give away the storyline so I'll leave it to you to enjoy.
Confounding factors: I'm enormously bias due to my immense love of X-Men and all affiliated products ;)