The movie was certainly the most "comic booky" of all the X-Men films so far. For me, that wasn't a problem. I cheerfully accepted the grandiosity of the Quicksilver sequence based on the 1980s setting: it was the decade of excess. Some of the sequences from the final battle were similarly over-the-top, mainly from the Horsemen, but I guess you could rationalize that by the enhancements that Apocalypse provided them. I like that they chose to include Archangel, but the original comic story of his fall tugged some emotional strings that weren't by this movie due to lack of depth for his character.
I was really hoping Charles (and maybe Raven) would die somehow, that Apocalypse would win, and that Magneto would realize that Charles' dream of human-mutant peaceful coexistence was worthwhile. By the end of the movie, he could have then led a broken X-Men team, and the movie would have set up an Age of Apocalypse film. However, what we got was a solid, above average "inbetweequel" (Singer's words) that presumably sets up the next period piece X-Men movie. As good if not better than First Class, but several notches below the stellar Days of Future Past. Totally agree with CoGro, the critics are definitely unfairly panning "Apocalypse," as it is not a turd like "The Last Stand" or "X-Men Origins." Some people were probably expecting the final chapter of a trilogy, but the filmmakers obviously have spurned the three-act narrative structure with these prequels. And that's not really all for the worse.
I think the Phoenix story arc, at least the original, cannot be done justice in film, but it would be cool to be proven wrong.
Also, it could just be my eyes deceiving me (I was tired when I saw the movie), but it seemed that they made up Rose Byrne to look older in this movie, which was set 20 years after First Class, but they didn't really do anything to age the other actors, namely Fassbender and McAvoy. Outrageously misogynistic if that was indeed the case. I've seen her in a lot of recent movies, and Dormé still be smokin'.