I just got back from seeing this.
All in all, it wasn't as bad as people are making it out to be. On the other hand, it wasn't as enjoyable as the first one, which is too bad.
At time, it seemed to just devolve into silliness - in a bad way. Not to mention, many of the Transformers seemed like they were created for a single scene.
And don't get me started on the practice of killing off characters just to bring them back to life. It was pointless melodrama when Star Trek did it to Spock, and it's still just as pointless with Megatron and Optimus Prime.
Overall, the most disappointing aspect was Starscream's role. In an interview with Michael Bay, he stated that in the original script (for the sequel) more time was spent on Starscream. Specifically, how he tried to lead the Decepticons himself, and how he viewed the fued between Megatron and Prime as pointless bickering that was wasteful for their species as a whole.
Through successive rewrites, all of that got tossed.
I've got more critiques, but why bother? In the end, it's an enjoyable movie that nevertheless isn't as good as its predecessor. Take it for what it is, and move on.