I'd probably put it in the A-/A range as a film. Strong production values all around, and the 60s period setting is tremendously fun; it's not surprising that superhero films don't do more of this, but it's refreshing (and there's another one coming up in a month or so).
The film's plot managed to surprise me in a number of instances. Some of them were just as simple as killing off actors that I didn't expect them to get rid of so quickly, as with Oliver Platt.
Character-wise, there are basically three tiers:
1. Professor X, Magneto, Mystique (surprisingly)
2. Beast, Moira
3. Everybody Else
Tier 3 is fine as action figures, but there are occasionally significant moments that hang on them which are a bit of a stretch - Angel's defection would be the most obvious instance of this. But then, having more characters than can be managed effectively is fairly in keeping with X-Men comics.
Everybody cites Fassbender as the standout, and he is indeed quite impressive, but I'll just chip in for Jennifer Lawrence, who impresses me more and more. I suspect she's at the start of a long and successful career. McAvoy is also very good as Xavier, though he strikes me as an actor who is perpetually waiting for the film where's he's allowed to be the main event; even when he's notionally the lead and is very good, he always ends up being overshadowed by his (often female) costars (Keira Knightley and Saorise Ronan, Forrest Whitaker, Angelina Jolie, Helen Mirren and Christopher Plummer, etc.).