tl;dr: It's very good. Gripping plot, great direction, mostly very strong acting, brilliantly affecting characters and outstanding treatment of themes. Go see it before I run out of superlatives.
Firstly, allow me to state my biases: I don't like the X-Men. Sure, the animated show when I was young was great, but the movies came around and put me off of them. The first was hardly the worst superhero action movie ever, but it's weak characterisation, bad dialogue and general mediocrity failed to endear me to it. The way it slobbered over Wolverine too, like a fan girl's gushing, annoyed me. If you like Wolverine then you'll be more inclined to treat it favourably. I don't like Wolverine, at least not as he comes across in the films.
Admittedly, the first is probably the strongest of the trilogy. To criticise the other two would be to regurgitate my feelings towards the first movie, as it is pretty much the same faults that dragged them down for me, too. I didn't even bother seeing the Wolverine origins movie, and popular concensus speaks well of this decision. So when another X-Men prequel was announced, naturally I was rather less than excited. Then the reviews started pouring in, and suddenly expectations were raised. Could this be as good as I was hearing, good enough to sit along the top echelons of comic book cinema?
X-Men: First Class starts off on a bleak note. In a scene straight from the first film Magneto is seperated from his parents during the holocaust, only to try and rip the metal fence around the camp down. A man spots him, a man impressed by the display of magnetic power, and summons him to try and repeat the effect. When the child is unable to, he summons Magneto's mother and threatens her. Seeing the child flailing impotently with his great power is a very stark way to start the film, but it never sets the tone so much as provide a strong under pinning to Magneto's motives in the film.
By contrast we see the posh and priviliged Xavier confronting an intruder in his own kitchen. It is quickly revealed that the child already has considerable control over his powers and is an intelligent and competent youngster. He is also compassionate, and it is a very complete young man that we see before our eyes. Soon enough, however, we see him emerging into adulthood, and it is clear that something has been lost. He uses his intellect to impress girls and spends his time drinking.
Our third protagonist, a female CIA agent who exists mainly to drive the plot forward, is then seen sneaking her way into a meeting of mob leaders, shadowing an important military figure. Therein she witnesses our antagonists: a gang of mutants playing an international game with massive stakes. Considering that the plot itself is the least compelling part of the movie is not a criticism of the plot itself: it's a fun superhero movie with more than a touch of James Bond style espionage. An international thriller that is married to the thematic ideas and character growth adeptly. More than anything else, it feels like the events of the plot really do matter and this leads to an exciting and suspenseful experience.
Really, what drives this film is the love that the protagonists have for each other as their underlying ideological differences tear them apart. Many of the highlights of the proceedings occur as Xavier and Magneto get to know each other, and find themselves to be personally and intellectually very well matched. This film is a massive tangle with four character arcs as well as the international conspiracy thriller element, and as such the way that development happens succintly yet with an organicness that lends weight to the development. Mystique and Beast, too, get a prominent role, and whilst their respective character arcs don't have the dramatic impact that Magneto and Xavier's do they certainly add value to the proceedings.
It's in the thematic conversations that take place between the characters, each one representing different ideas within the subject, that sets this movie apart as above the cut. It's especially remarkable for just how balanced a look at the ideas are given. There is no "right" side here, and throughout the film you're never quite sure which side is right, nor as to which of the two leads really is the hero. Whilst Magneto is certainly overly aggressive and too rigid in his approach, Xavier is likewise too goody-goody and unwilling to act. As you watch you'll find yourself symathising with both side.
Really, it won't become clear just how great a movie this is until we reach the finale. Herein the inherent tragedy of the story suddenly becomes clear, and the aura of inevitability that surrounds the events only serves to further strengthen the events. It is also remarkable, in that we know how this story is going to end, yet it still throws some surprising twists at us. When everything comes together, it does so beautifully.
The way that character growth, plot developments and thematic concerns develop in tandem with each other is a joy to watch. If there had not been such a strong interlinking then the film would likely have fallen apart, overburdened by plotlines and silliness, but it is overall a well crafted story. Maybe even a match for The Dark Knight? Well, I intend to compare the two directly soon.