It's kind of surprising for a guy like Michael Bay to actually take a lot of the criticism he got for Transformers 2 and actually take some of that criticism to heart. I will give him credit that the third installment is better than 2, but it's still nothing more than special effects scenes strung together by images of people running for their lives and shouting.
The people in this are so inconsequential to the story and what's going on onscreen, you can't help but wonder why they even bothered to write them into the film. Shia Lebouef's character is now trying to find a job in Washington, but is stuck working the mailroom, with his creepy boss (John Malkovich) hitting on his girlfriend (some Victoria Secret model taking over for Megan Fox, who got fired by producer Steven Speilberg when she compared Michael Bay to Hilter on set). All of this is so incidental to the action and story, it's almost abstract.
The first half of the film is a bore - for the reasons mentioned above - they really are just burning time, setting up some events in the film to give us the excuse of seeing digital buildings topple over in the Chicago landscape. The movie has a lot less juvenile humor (robotic body fluid jokes are reigned in substantially) and Michael Bay resists going into his usual broad stereotypes that populate his films (Sassy black lady giving attitude in bureaucracy situation - those two really offensive robots from the last movie are thankfully nowhere to be found), which lends the movie a bit of credit to it when the mayhem starts from the last 90 or so minutes.
Don't expect much, and you'll enjoy yourself with this. I went in to see this with the only expectation that I'm wanting to see a movie where vehicles transform into giant robots and fight each other and it delivered that - nothing else.
Edit - yes, it's worth seeing in IMAX 3D for the spectacle of the last 90 minutes.