At this point I like to summarise the plot of the film. I'll give it a go, but the fact that I can't really remember it should tell you something about Revenge of the Fallen. Since the first movie Optimus Prime and growing gang of Autobots are helping the US keep Decepticon visits on the DL. A plot device falls out of Sam Witwicky's (Shia LeBeouf) jacket and information gets downloaded into his brain regarding the location of 'the matrix', a device that will activate another device and... destroy the planet? I forget. The Autobots must protect Sam and his loved ones (plus comic relief college roomate) from the Decepticons, who want the device. Explosive metal clanking hilarity ensues!
The plot of Transformers 2 is far too convulted to make much sense. When a film is two and half hours long and still doesn't manage to explain the story adequately, there's something wrong. The plot's in there somewhere but it's drowning in a sea of too many robots, pointless college comedy and unnecessary subplotts. Seriously, I don't care if Sam says 'I love you', just explain to me why there's a bearded Decepticon hatching eggs in space. Too much to ask?
There are some highlights - Bumblebee still makes me sad that you can't hug robots, the giant hybrid Decepticon is pretty badass (but too easily defeated), and all the returning characters are welcomed (especially the delightfully weird Agent Simmons played by John Tuturro). It's funny in places, and the fights are kickass, but where the film falls is its clutter. The Cylonesque Decepticon was just stupid, and most of the Sam's college excursion felt like it wanted to run away from home and make its own movie.
Revenge of the Fallen is great mindless fun, and if you like your mindless:fun ratio favouring the mindless part, you'll have yourself a whale of a time.