I give it an A overall, because of largely one reason:
Unlike the original X-Men film trilogy, this movie makes the characters feel comfortable in their own skins. It plays out very naturalistic, with a sense of confidence. The X-Men trilogy was always force and awkward - always a bit too aware that it was trying to "sell" the mainstream audience on these guys, who have Strange Powers, and Sort of Wear Costumes, and may be secret heroes, etc, etc.
I'm not going to add to the long reviews here, but I will point out my only two noteworthy issues:
1. Emma Frost was miscast with an actress who seemed not to respond with emotion or even concern to anything going on around her. The White Queen is a character that is supposed to "steal the scene" when she walks in the room. This performance was utterly flat and pedestrian.
2. Erik suddenly transformed into Evil Mutant Brotherhood Magneto as soon as he stepped out of the wreckage after killing Shaw. It was too sudden. He should have been more angry and panicked, dealing with the missile attack in a more emotional way and with more difficulty considering all he'd just been through. He should have then pleaded, not commanded, people to go with him, get away from the government, go underground, because it was pretty sensible that with the crap that'd just gone down nobody was going to even believe their side of the story. Instead Erik starts blathering like a villain about The Humans and such - he hadn't even had time to develop his Homo Superior chick tracts yet.
Also, a thought on continuity. I don't really mind that we have a few direct contradictions such as Xavier already in a wheel chair, and Erik having already fallen out, when say the opening of X-3 shows a flashback to an older but mobile Xavier cooperating with Erik. Or when Xavier says Magneto helped him build the modern Cerebro. There are still two decades or more of X-Men stories to fill in the gaps. That includes the usual plot twists, such as Xavier having bouts of mobility, or Magneto deeming to work with Xavier again at certain points. If one went back and read the early X-Men stories they'd also appear to have impossible continuity errors when compared directly with a present day storyline - so much has happened involving bait and switches, face heel turns (and back again), and resurrections from the dead.