I'll break down the pros and cons after the break, and there will be spoilers, so consider yourself warned.
Things that worked
Hands-down great action. Action was the biggest problem in Revenge of the Fallen, which feature one great forest battle, and then became incredibly confusing. As far as I'm concerned, Dark of the Moon solved this problem. The first set piece takes place on freeway and features some brilliant transforming effects and actually good 3D. And that means a lot, considering I don't like 3D.
The Transformers are really characters now. I'm not sure what it was exactly, but Bumblebee is somehow more than just a cool car, he's now got enough of a personality that I actually was worried he might die at one point. Additionally, Optimus and Ironhide are both given enough to make them matter, and the new Sentinel Prime is at least better than last film's Fallen.
Some great supporting characters. Firefly alum Alan Tudyk turns in a great supporting performance (as a butler to whom there's more than meets the eye. John Malkovich, Frances McDormand, and a voice role by the great Leonard Nemoy. Oh, and John Turturro is back for whatever that's worth.
All-is-theoretically lost. There's a moment around the middle of the film where the Decepticons plan has seen Sam Witwicky (Shia LaBeouf) turned into a spy, the Autobots supposedly destroyed in a shuttle crash, and the Decepticons invade Earth. Now I never actually thought the Autobots were dead, and this being a Michael Bay movie, I knew the good guys would win, but there was a short section of the film where I couldn't see an easy way out for the characters, and that's something the other two films didn't manage.
Things that almost worked
Human villains. Early on, we learn that Sam's new girlfriend Carly is working for some rich dude (Patrick Dempsey), who's a jerk and also works for the Decepticons. The idea of giving the film a human villain is good. Unfortunately, the execution is poor, and Dempsey never becomes convincing threat.
Unconvincing betrayal. The whole "Sentinel Prime is really a bad guy" line was kind of cool, but I didn't really care since I'd only just met the character. I was actually expecting him to die pretty quickly, but either way, I wasn't surprised.
Things that didn't work
Easy resolution. I suppose I shouldn't have expected more, but shortly after setting up the dire situation, Sentinel Prime, Patrick Dempsey, and Megatron conveniently explain their entire plan in front of Carly, taking care to point out that one
So now their easy to kill. Well, here's the biggest spoiler. Both Megatron and Scarscream die in this movie, and that would fine, if they weren't so damn invincible in the previous films. Basically, Megatron gets his head ripped off after a thirty-second fight with Optimus, begging the question, "Why didn't Optimus rip off his head in the first movie?"
Too many Transformers. I'm sure I'll be able to figure out who's who when I look at the Wal-Mart toy rack, and I get that it's a side-effect of a franchise built around a line of toys, but there's about three new characters that only get about five minutes of screentime between them and all look basically the same.
Half-an-hour too long
The new girlfriend. I don't know whether to blame this on new actress Rosie Huntington-Whiteley, or poor writing, but the end result feels like they shoehorned Sam's girlfriend change at the last moment, cut anything important she was going to do, and turned her into a generic damsel-in-distress. There's a scene in the final battle (I think's actually this scene) where the film cuts to her for five-to-ten seconds. I kept waiting for her to do something, and then it cut away. To be fair, she does look good.
Running time. At two-and-a-half hours, the film is thirty minutes too long, and there's a few things, like Sam's little robot friends, that could be cut completely without changing the film in the slightest.
So there you have it, the things in the top section outweighed my frustration with the others, and this film was two good hours of fun.