I love this series (despite a few missteps in X3 and the Wolverine movies) and with Brian Singer returning to the director's chair have been very much looking forward to this new one.  I'm not sure it's quite as good as the extremely positive reviews make it out to be but I certainly enjoyed it.  It's a major improvement over X3 in everything but the action, and overall I'd say about on the level of First Class, though more consistent in quality than that more up-and-down effort.  The first two remain my favorites - despite the smaller scale I think they still have the most dramatic storytelling of the series - but this latest was good enough to have me looking forward to the next one.

On to the spoilery stuff...

Quicksilver stole the show.  I came in to the movie expecting not to like him, but his quirky adolescent charm won me over from his very first scene.

This is the first time in the series I've really been impressed with Mystique as a character.  She was bad-ass when she needed to be, her conflicting role in the story came across well, and the story premise revolving around her was clever.  And this is hardly the first time someone has commented on Jennifer Lawrence's looks but her non-Mystique scenes... wow.

Loved the ending.  I'm not sure a genre film has played on sentiment and series nostalgia so perfectly since Return of the Jedi.

The post-credits teaser was a nice touch.  I'll be curious to see how they handle him in the sequel.

I liked it lots, certainly more than First Class.  As with most superhero sequels, Days has the benefit of moving past the 'origin story' roadblock of its predecessor, and getting right into the meat of the action.  Kudos, too, to Singer for taking the overarching plots of these films (even the ones he didn't direct), and being able to turn in something that both retcons and respects the continuity in a satisfying way.

I was worried about the overall look of the Sentinels when I saw the trailers -- and while I still think they're a bit on the smallish size, their unrelenting ferocity more than makes up for that shortcoming.  Seriously, they TEAR SHIT UP here, and it is thrilling.

McAvoy and Fassbender are still the highlights, but I really enjoyed Nicholas Hoult's more proactive role as McCoy/Beast this time around.  Adam, I gotta disagree about Mystique.  Despite her importance to the plot, I found Lawrence's stoic, one-note portrayal of the character to be bland when compared to the sympathetically vulnerable Raven we got in First Class.  It made her...... crucial turning point when she decided to not go through with the assassination a bit hard to swallow.

And Adam, what did you think of Ottman's score?

It's funny, this is a point you often hear in reviews of comic-book movies and other genre fare, and one I completely disagree with.  Personally I enjoy origin stories and find they often offer more meaty character interaction than the sequels.  Examples:

Spider-Man > Spider-Man 2 (I know I'm in the minority here)
Amazing Spider-Man > Amazing Spider-Man 2 (the love story among other things)
Star Trek > Star Trek Into Darkness (far more interesting and endearing characters in the first)

Even with the first two X-Men films, I'm inclined to say I'm more impressed by the character interaction in the first movie (the Logan-Rogue and Xavier-Magneto scenes are exquisite), it's just X2 completely overshadows it in scope and effects.  And in the latest two movies, I'm not sure any of the character development in DoFP has the potency of the Xavier-Magneto dynamic in First Class, it's just Singer has much more story-wise to work with in DoFP and handles it with better consistency.

Obviously there are exceptions (Empire Strikes Back > A New Hope, Dark Knight > Batman Begins) but in general I think origin stories get a bad rap.

Don't get me wrong.  I enjoy a good origin story myself, and I can confidently add Iron Man and Kick-Ass to the list of films where the first in the series is far superior to their sequels. (Agreed about '09 Star Trek as well, if we're talking non-superhero movies.)  By the same token, I think comic book sequels such as Superman II, X2: X-Men United, Spider-Man 2, Captain America: Winter Soldier -- and yes, The Dark Knight -- were better outings than their predecessors.  And not simply because of their larger scopes, bigger budgets, and better effects; with the world building already established, character interaction frequently felt more natural and self-assured, and narrative progression more focused and varied.

In the case of First Class, I think, as a prequel, the problem was that of predictability. To quote from said movie's thread:

"X-Men: First Class is not only a prequel to the popular cinematic series about Charles Xavier and his merry band of mutants, but it's an "origin story."  And therein lies the problem.  Origin stories, by their nature, are less about telling compelling tales than assembling all the pieces. In road trips, the journey matters more than the destination - in origin stories, the journey is irrelevant as long as, when it comes time for the end credits, everyone is where we expect them to be. It's difficult to surprise, tough to become immersed in a narrative when there's a checklist in the back of the mind of things that have to happen. This impediment is not unique to X-Men: First Class. It's a universal prequel problem, and one of the reasons why superior prequels are hard to find."

The first Amazing Spider-Man was a particularly egregious offender, by virtue of being a reboot that arrived much too soon. Minor change-ups to the plot aside, they were essentially telling the same exact origin story that we'd gotten only ten years prior. (And rather inadequately, it can be argued.)  For all the flaws that The Amazing Spider-Man 2 has, it at least attempted to tell some semblance of a new story -- or at least a few twists on an old one.