I saw this movie after reading a few (non-spoiler) reviews which rated this really highly, possibly the best superhero film ever. I don't know that I'd go quite that far (I'd probably give that honour to The Dark Knight), but I would definitely rate it in the top 5. The time travel stuff does fry your brain if you stop and think about it for too long, but if you adopt the Austin Powers approach ("I suggest you don't think about it too hard, and just try and enjoy yourself") then it's fine.
I'm not a comic book reader, so I knew nothing of the source material. I subsequently read about some of the differences they made, but they all seemed either necessary or unimportant. Logan/Wolverine's role is pivotal, but it was nice to see him less of the action man and more of a silent narrator. While it's always fun to watch chaos and mayhem ensue when Wolverine's claws come out, it also helped to embed the film with some "realism", showing him working as part of a team rather than rehashing his "man alone" schtick from the first three X-Men films. It also reinforced the idea that this was a "First Class" film with cameos from the "original" series, rather than the other way around.
There were so many great characters in this film - Raven/Mystique, Charles (the younger), Eric (the younger), and Bolivar Trask all stand out for me. I especially liked the way they portrayed Trask as the villain - it made a refreshing change from the previous villains of the earlier films, and seemed to borrow a little from the Game of Thrones storytelling style of avoiding simple distinctions of "good" and "evil" and instead showing a more nuanced interplay of issues. It probably helped a lot that Peter Dinklage was in the role, who does "nuanced" very well. Quicksilver was done really well too - the only major new character to be introduced, and they did it really well without distracting from the main story.
There was only one post-credit sequence - right at the very end of the credits, so if you like those stick around until the very end of the credits for that.