I don't think XMOW screwed anything up. It was after XFC that things got iffy, continuity-wise. And even then, after X-DoFP, XFC will be more canon than the other movies.
Well, Origins Wolverine screwed with the how of Logan's transformation and the how of how he lost his memory. It's implied heavy in the flashbacks in X-Men and X2 that he lost his memory during the procedure not from the damn adamantium bullet. That's just the big stuff. There's other small things.
And yes, after Days of Future Past, I know First Class is in continuity but I'm going to go out on a limb and say that once Days of Future Past is released, we may not even have a need to watch First Class as well.
This is just a guess on my part but essentially, Days of Future Past is X-Men 4. Just based on the post credit sequence of the Wolverine, we're getting X-Men 4 that just happens to take place majority in the past where we get to see the major players in their younger form. Again, I don't know, but we'll see where those younger characters are at at the beginning of the film to see how everything will fit together.
But, at the end of the day, the end of Days of Future Past will create a cohesive timeline for moving forward with the X-Men franchise. That's the purpose of the film, get everything straight and exampled so that, moving forward, they can do all types of X-Men films.
That's why the film is being touted as this big event. It's meant to straighten out everything...which it can never really do.
