Over the last month, I've re-watched every entry in the series that Fox has produced since "X-Men" in 2000 as my kids worked their way through the films for the first time. What I found most interesting about the revisit is how my reactions to the films as they were being released and my reactions to them now aren't really the same. Each film was on a specific point in the larger continuity of comic book related movies, and the only way to judge them was how they stacked up to everything else that was being done in the genre at that time. Now, though, looking back at them all is illuminating, especially since it feels like "X-Men: Days Of Future Past" is the end of everything up to this point and the beginning of whatever comes next.
One thing has been very clear since this series began: while they are drawing on comic continuity for suggestions, they have not been bound by any particular rules of adaptation. In fact, they have almost exclusively altered things. I don't think you can point at any particular run of the series and see a direct correlation between what's onscreen and what was on the page. Sure, "X2" and "X-Men: The Last Stand" nodded to the "Dark Phoenix" storyline, but barely. I would find it a little ridiculous to actually call those films an adaptation because of how loosely they line up to the motivations or the characterizations of the books.