A bigger budget this time.  And a better movie in most ways.  This is the one that best captures the adventure and thrill that superhero comics are all about.  If you're not looking for an art film, but want a solid, satisfying, meat-and-potatoes superhero adventure, you won't find a better one than this.  There are a few bogus moments, as in any film of this type, but they're pretty easy to get past.  (But unfortunately, the one sequence in the film that most needs the audience to avoid thinking too much about the reasons for what happened, is the climax.)  There are also some terrific action pieces, much more impressive than those in the first film, particularly the opening sequence.

Colonel Stryker, a "military scientist" who has reason to hate mutants, has come up with a way to control the minds of those he captures, and thereby turn the powers of mutants against their own people.  His attack on mutant-kind forces Professor Xavier and Magneto, the enemies of the first film, to join forces...

The Stryker character is a bit too much of a James Bond villain, but for a true comic book plot, sometimes that's exactly what you need.  Halle Berry as Storm is once again a weak point in the cast, this time turning in a performance inconsistent with how she portrayed the character in the first movie -- a different accent, for instance.  But most of the rest of the cast of mutants is strong throughout, in major and minor roles alike, and Hugh Jackman as Wolverine is, more than ever, the rockin'est tough guy in comic-book-film-dom.

One awkward bit is that the film has a high body count yet very little blood.  They had to keep the blood out to be safe for the kids to see.  Yet some of the scenes, if realistic, would be drenched in it...

Someone once said that Lawrence of Arabia may not be a great film, but it's certainly a grand film.  Likewise, the third X-Men movie may not be great, but it's certainly... big.  Maybe you could call it grand.

X-Men: The Last Stand is the kind of a movie that you might enjoy every minute of while watching it in the theater, but then start noticing lots of problems with when you think about it afterwards.  Since this is no less true of a lot of the other top superhero movies, such as Spider-Man 2, these flaws don't end up costing it anything in the cape rankings.  It does lose a bit of depth relative to the first two X-films... but in compensation, it offers far more action.  There are, for instance, superpower vs. superpower fights on a scale never seen before.

As others have pointed out, in a film series, it's at the third film that they generally stop thinking in terms of crafting a single work and start thinking in terms of ongoing assembly-line production.  Generally this involves a decrease in the talent and resources behind the camera.  Often the third film is the one where the original director is replaced.  Occasionally this ends up actually improving the results (e.g. the Harry Potter films), but usually the results are as in the Batman or Superman serieses: the third film is crap.  In this case, sure enough, the director is replaced.  Bryan Singer, who made the first two films what they were, is replaced by undistinguished journeyman Brett Ratner.  But it's okay; Singer has established the style well enough so that the differences are subtle.  There's a bit less depth and more action, but they don't dumb down the story.  And they most certainly don't make this just a generic middle- of- a- series episode; the film company decided they wanted a real capstone to the trilogy, so there are major events that leave permanent changes in the cast of characters.  People die.

Unfortunately, both Hollywood and the comics industry have a terribly difficult time with letting anybody stay dead...

The plot combines two stories into one.  One is the "Dark Phoenix" saga, in which a certain character who "died" in the previous film turns out not to be dead, but who when restored to life is no longer able to be one of the Good Guys, instead becoming a danger to everyone.  The second story is the main part of the film: somebody comes up with a "cure" for the mutation that makes the mutants different from regular human beings.  In public, the government is making it available on a purely voluntary basis, for mutants who want to rejoin mainstream society.  And many are glad to have the "cure".  But in private, the government is turning the cure into a weapon, knowing that certain mutants, notably Magneto, will react with violence to the idea of mutants being de-mutated.  Of course, the government's weaponization of the treatment provokes exactly the upheaval they were trying to guard against... such is the way of arms races.

Some have said that this movie tries to do too much at once, that it crams in too much story and too many characters.  Some fans, for instance, thought the "Dark Phoenix" story deserved to have its own film.  I disagree; to me, the sprawling nature of the tale and the large cast are strengths, and the Phoenix tale isn't enough on its own.  (I might as well say here that one weak point of the first two films is that the Scott/Jean/Logan love triangle never made itself very believable.  Never had any juice in it.  That was okay as a minor side issue, but it would definitely undermine any attempt to pump up the Phoenixery any larger.)  I kind of wish we got that kind of wide dozens- of- characters story more often in genre movies.  Big events in real life never center on just a few characters; there are always dozens of peripheral players.

I don't want to nitpick away at too many of the shortcomings, but it has to be said that it hurt the film when Halle Berry demanded a bigger part, and the producers decided to keep her happy.  So she gets lots of screen time and lots of battle action which would have been better spent on someone more interesting, such as "Nightcrawler", played so vividly in X2 by Alan Cumming.  He's entirely absent from the cast this time!  Also, they definitely play the "he's not dead after all" card one time too many.  They were getting away with it right up until the very last scene, after the credits...

And for the characters who do die, the movie doesn't do much to make you care about them.  It depends almost entirely on whatever feelings you have for them are left over from the first two films.

The rest of the complaints I will keep quiet about, though there are lots more, because I definitely do not want to denigrate the film.  It fulfills the role of a big-ass action blockbuster very well -- in fact, it makes the first X-Men movie look rather petite and dainty -- and it still respects your intellect.  Even nowadays, when dumb loud action movies are definitely getting less dumb than they tended to be before the X-Men series started, that's still an accomplishment.  And despite the evidence of decline relative to its predecessors, this is still a better and, dare I say it, truer story than we've seen from Spider-Man or Batman* or any other successful superhero fim franchise.

* at least up until The Dark Knight