This is no simple topic, as far as I am concerned. Let's start with a few movies he's been in. You may wish to consult the wikipedia entry before submitting your vote / thoughts.
http://en.wikipedia.org/wiki/Matt_Damon
In my opinion he is decent. I thought Good Will Hunting was a great watch. I'm a huge fan of Robert Ludlum novels and so really liked The Bourne Identity (although the original is millions of times better). Also the fact that he worked with Kevin Smith is a major boon for him.
So :