Along with 1998's Blade, the original X-men helped to kick-start the modern superhero genre. While a few hallmarks of the original X-films (more serious focus, toning down of the colorful comic book elements) saw themselves reflected in movies like Christopher Nolan's Dark Knight Trilogy, there was very little else to the early films of the franchise of note, other than giving screen appearances of varying quality to an increasing roster Marvel mutants. As the franchise hit choppy waters with X-men: The Last Stand and X-men Origins: Wolverine, Fox looked for ways to breathe new life in to the franchise.
This paid off with the pretty good The Wolverine from last year, and the very good X-men: First Class from 2011, but interestingly enough it's this year's Days of Future Past that not only restores the franchise's financial success and brings it to new critical heights, but finds a unique twist that both makes Days a compelling film, and also gives the franchise something to truly call its own in a continually crowded field.
In creating one of the most "comic book-like" films ever made, this franchise has actually retconned it's own history.