He's had a couple pretty darn good years recently (cumulative numbers are quite good, Orton/Denver weren't great, and the transition to the Rams, who were fucked, was obviously a bit rough).
The man has amazing hands. AMAZING. He had some concentration issues when he was younger, some maturity issues, but you can't blame him for all of those down years. Washington was shit, and he missed significant time with injuries 3 years in a row.
He's proven that he's a quality player, and he's been healthy the past two seasons. Probably figured out how to really take care of his body. Not a very long contract, easy to terminate, so he's going to stay motivated. Plus he'll have a shot at the Super Bowl every year.
He may just kill it with Tom Brady, in a Josh McDaniel's offense. Pats pass attack is going to be ridiculous, his numbers largely depend on distribution (lots of options), but to me this looks like a huge move for the Pats.
These are some ridiculous hands. An elite skill-set, really.
http://www.youtube.com/watch?v=VnvyQuLPKDM&feature=related