From creativescreenwriting.com's newsletter CS Daily:

Simon Kinberg & Zak Penn
The X-Men return with a new creative team, a resurrected teammate, and a mutant cure that could allow them to live without fear of intoleranceor bigotry. Unfortunately, the depth andcomplexity of the previous films is replaced bysound and fury (and the occasional one-liner),leaving us with an entertaining popcorn flickthat nonetheless fails to live up to the highstandards set by the series.
Sometime after the events of X-Men 2, the world seems to a marginally better place formutant-kind. Hank McCoy, a blue-furred graduate ofCharles Xavier's School, serves the President ofthe United States as Secretary for MutantAffairs andMagneto is on the run from a governmentdetermined to bring him back into custody. However, the X-Men stillreel from the loss of their companion Jean Grey, whosacrificed herself to save them at the end ofthe lastfilm. Like C-3PO, though, the X-Men are made tosuffer, and trouble soon rears its ugly headfrom two different directions: a so-called"mutant cure" thegovernment has developed, and the return of JeanGrey, now mentally unstable and possessed ofterrible,apocalyptic power. Director Brett Ratner andwriters Simon Kinberg and Zak Penn do a passablejob of crafting a fast, funny action flick,which nonetheless feels every bit as rushed as itsgotta-beat-Superman production schedulewould suggest. Despite somepopcorn entertainment value, the scripttries to do too much with too many, substitutesone-liners and flowery speechmaking forcharacter development, and sabotages itself withlast-minute revelations that betray the darkermoments of the story.

Last Stand features two major storylines-- the mutant "cure" serum and Jean Grey'sreturn and subsequent turn to the Dark Side -- either of which could easily sustain an entire film on theirown. Unfortunately, by choosing to try to coverboth, neither story gets the service it is due.The concept of a mutant cure raises questionsperfectly suited for the type of intelligent, metaphorical storytellingthe previous X films have delivered. Isour identity defined by our genetics? Is it wrong tosacrifice who we are so we can fit it? These conundrums aretouched upon by characters like Rogue and Beast,but because there's so much going on, theimplications of that choice are never fully explored. Nor isJean Grey's story given its due. The "Dark Phoenix"saga is one of the most beloved and revered storylines in comic-dom, a story that should be heartbreaking if realized properly on screen. Instead, Jean's inner struggle is sketched only in the broadest ofstrokes, and the tragedies associated with her fall intodarkness pass by with little or no emotional impact. When screenwriters Kinberg and Penn spoke to Creative Screenwriting about the film, they insistedthat the heart of Last Stand was the Jean Greystory. I wish that were the case. Instead, we get a fewmoments of inner turmoil, some lazy writer'sshorthand (she's horny, she must be evil!), anda wholelot of people talking about how dangerous she isnow. The film is so busy, so packed with event,with manyelements worthy of further exploration justtossed in offhandedly. Sure, fans of the comichave been waiting for ages to see the giant,mutant-hunting robotic Sentinels up on the bigscreen, but if the option available is aten-second sequence of a pair of floodlightshanging from a lighting rig 30 feet from thecamera, I'd prefer not to see them at all. Thelaughable Sentinel cameo that appears early inthe film is emblematic of the film as a whole,throwing in concepts just for their own sake. They aren't done justice, they just take upspace, leaving no room to breathe, no room forcharacter, and elements that could have madefor several exceptional X-Men films isinstead compressed into one mediocre one.
Which is not to suggest that the film isn'tenjoyable. It's fun. It is often intense, fast-paced, and the writers manage to take advantage of thesesuper-powered characters in clever ways, makingfor some truly memorable action beats (the fightbetween Wolverine and a mutant capable of regrowing hislimbs is not to be missed). But after you leave thetheater you'll likely either completely forget about thefilm or else, like me, find yourself turning it over inyour mind and realizing just what a flimsy creation it is.The writers have given us spectacle at the expenseof character. Storm takes the forefront asleader, but her acceptance of that mantle never feels organic. Roguish Wolverine now feels like Wolverine aswritten by Danielle Steele, with much of the edge andunpredictability of his character replaced by unconvincing sentimentality. None of the newcharacters are given enough time to shine, and what time they are given means we spend even less time with the ones we've come to know and love from the earlier films. Poor Angel gets basically threescenes, and one of them of the deus ex machinavariety. The previous films have picked one or two characters as anchors for their emotionalthroughline -- Rogue and Wolverine, as outsiders, were the natural choices for the first film -- but Last Stand never seems to recall this lesson. This couldeasily have been Jean Grey's story, or Rogue's, orBeast's. Instead, the story is everyone's and no one's,and that lack of focus cripples the film from the start.
Spoiler warning!
The most unforgivable sin committed by the script, however, is only revealed in the closing minutesof the film. Without giving away specifics,suffice tosay that this film has a high body count, bothphysicallyand narratively. Major characters are killed,maimed, or stripped of their powers. Part of the problemwith these decisions is that they're not giventhe emotional heft they deserve. None of the deaths affect the other characters the way they should -- one weepy graveside speech does not an honestcharacterization make, folks. Nevertheless, none of the missedopportunities for emotional resonance offendsnearly as much as what happens during the finaltwo scenes (one located after the closingcredits, FYI). During those scenes, onecharacter's death and onecharacter's lost powers are both negated in acareless and offhanded fashion that completelyundermines what little emotional impact thescript generated from those events in the first place. I realize this is acomic-book movie. I realize that characters inthe X-Men comics regularly die,resurrect, return as clones, fake their deaths,and have their minds transferred into the bodiesof scantily clad Asian martialartists…but this ain't the funny books.The series so far has made great efforts toground the X-Men's fantastic elements against agritty reality. Jean's return from death shouldbe a unique occurrence, and should be earned byall the terror that results from her return fromdeath's door. By undercutting two of thebiggest narrative risks of the film, the writerseffectively undercut all of them.
People will always remember the freeway chase from Terminator 2, but the reason that movie is a classic is because the characters of Sarah Connor, John Connor, and the Terminator were so wellcrafted by Cameron. People will also remember the ladder-truck chase sequence from Terminator 3…but that's about it. Unfortunately, X3 falls into the same camp: a fun,forgettable popcorn film that could have -- and should have -- been so much more.