It completely contradicts the last two movies.

Rating: 5/10 (4 June 2011)

Which is the only reason I don't give this a perfect ten. MacAvoy and Fassbender are superb as early day versions of Charles Xavier and Erik Magnus Lensherr. And, even the transposition of Darwin and Tempest from their early 21st century comic book appearances, to the Cuban Missile Crisis, I can deal with.
But, what I cannot abide is needless rebooting with no regard for the memory of the older movie-goers, or credit for their intelligence.
Emma Frost was ten years younger in X-MEN ORIGINS: WOLVERINE (which was set mostly in the 1970's) than January Jones' portrayal, here. Of course, fan boys like me can easily ret-con that by saying: "Oh! Maybe the one in ORIGINS is this one's namesake niece. Maybe even her daughter! After all, Silver Fox did call her 'my sister.' Which could imply some kind of adoptive sibling relationship."
But, not so easy to ignore is the fact that this prequel depicts Xavier being accidentally crippled by Moira McTaggart. Whereas, in the flashback prologue of X-MEN: THE LAST STAND (set circa the 1980's), he was seen as perfectly ambulatory. _Walking_ into the Grey family's residence, to meet adolescent Jean, side-by-side with Lensherr!
How are you going to explain that one away, Fox and Marvel? A bitter, time-traveling Romulan from the 24th Century? I don't think so.
In short; the movie wasn't bad. But, in this case? "Not bad" is just not good enough.