I found Caligula by Sam Wilkinson to be a good coverage of the revisonist view. Not sure how far I believe it, but he does present a good, reasoned argument that makes a lot of sense and has evidence to support it.

According to this theory Caligula wasn't mad - he was too good. He looked to be another Augustus or Caesar, i.e. a person who would take power further from the senate and shake up the Roman world. The senate didn't like this so he was dealt with in the same way as Caesar. As he was very popular with the plebs a massive cover up was set in motion; the mad dog was put down for the safety of the populace.

It's certainly made me a lot more thoughtful about the matter, less ready to accept the standard story.