All four of the major players here (Hugh Jackman as Wolverine, James McAvoy as young Charles Xavier, Michael Fassbender as young Eric/Magneto, and Jennifer Lawrence as Raven / Mystique) do fantastic jobs. They are who they are, who the story needs them to be, and who we expect them to be. Of the four, Lawrence shines the brightest I think because the previous films haven't given her the chance to be a fully authentic adult motivator. Here she is her own person, caught in a three-way tug of war among Charles, Eric, and her self-declared path.
(ETA: Jim Hines has several good comments in his (spoiler alert) review of which I wanted to highlight the "more Mystique, less angsty white dudes" comment particularly.)
The premise of the movie is that the human-vs-mutant war is in its final days. The humans have honed their Sentinel hunters into unbeatable machines and have killed or imprisoned all mutants, those who bear the recessive gene that might someday give birth to a mutant, or just simply sympathized with the mutants. The remaining mutants concoct a plan to send Wolverine's mind back into his 1972 body. There, he must convince Charles and Eric to patch up their feud and cooperate to stop Mystique from killing Dr. Trask, whose black weapons program will produce the Sentinels.
In the history of this film, that assassination is Mystique's first killing and it sets her on a murderous course as well as convincing humans that they need the Sentinels for protection. By saving Trask, the future Logan can save Mystique and prevent the disastrous war.
If this sounds tense and interesting then you're thinking the way I was. Unfortunately, Singer can't be satisfied with just this so we get several drawn-out scenes of future mutants battling the Sentinels. It's fan service, and just drags out the film. Ororo gets to fly and throw lightning, and you get to see Colossus and Iceman and so on. I guess the special effects guys needed more to do, but I found it boring.
Still, a good summer fun film, and do stay the end of the credits.