A bit flat, and certainly not the best of the X-Men films, but for causal fans and moviegoers, it'll do just fine.
Alas, my biggest issue with it is to do with the villain. Apocalypse is, in the comics, perhaps the single greatest and most powerful villain the X-Men ever faced. In fact, so much so that it spawned the mini-series 'Age of Apocalypse,' quite easily one of the best storylines ever.
The version shown here in this film is a pale, limp version of what he should be. In fact, had the writers really given it some proper thought, he could easily have been to the X-men what Thanos was to the Avengers. They could have easily created two movies out of this, and raises the stakes far higher. Google Age of Apocalypse for an idea of just how underused he is in this film.
Instead, they bring him in and wrap him up in one film, even relying on the crutch that is 'The Phoenix' to finish him off - because we haven't already seen this once before, apparently. The X-Men universe is so vast, and yet they just recycled material already touched on in X3 to give them an ending. The fact that they had to go further with Dark Phoenix....well.
As I said, a decent film in its own right, but knowing the character well means you know that it could have been far, far better. A huge missed opportunity, and one I hope Disney will rectify later on down the line. Apocalypse truly has the potential to be every bit the threat that Thanos was - hell, even his four Horsemen in the comics were behemoths in their own right (have a look at Holocaust and tell me you wouldn't want to see him on-screen).
Fingers crossed we see more of the X-Men - but done better.