When I reviewed 1st class 3 years ago I said " If you are looking for a ranking, it's certainly better than X-Men Origins:Wolverine and X-Men 3: Last Stand, but not as good as 1 & 2, though I feel it could have been". And I feel exactly the same could be said about Days of Future Past, it sits next to First Class and moves from better to worse scene by scene, before finishing about level I reckon.
It does some things better, it's certainly better paced than First Class, and it includes the best standalone set piece scene since Nightcrawlers assault on the Whitehouse. Which is high praise indeed since the first 3mins of Xmen 2 stands among the best openings ever to me, indeed X2s biggest issue was trying to match that opening 3mins for the next 2 hours and never being able to. Days of Future Past places its set piece in a much better location. DOFP also introduces enough new characters and leaves us wanting more, not less, which is always the best idea.
However it does lack the heart of First Class, and the great relationship building we saw in the first 3rd of that movie between Charles, Erik and Raven. This never comes close in that regard, how can it when Wolverine is the star (they really need to park him more, he's just too over-exposed). Also some of Mystiques scenes and indeed sfx seemed particularly bad, not sure what went wrong there.
But that aside it's a competent and fun superhero movie and well worth seeing. It certainly continues First Class job of getting Xmen back on track from the travesty that was Last Stand/Wolverine Origins.