I commute every day from Davis to the Financial District down Beacon to Kendall and across the Longfellow (I'm the guy on the chrome Bianchi Pista with a Russian flag Timbuk2 bag). Somerville's roads suck. Straight up. Potholes aplenty and manholes smack dab in the bike lane, one every block. Still, once I hit Cambridge roads, it's great (save the Kendall area itself). Once in a while I'll take Mass. Ave. when I don't feel like taking all the bumps. Ditto on whitelining in order to avoid doors.
As far as traffic goes, it's alright as a cyclist. Drivers here are indeed agressive and otherwise incompetent, but if they see you, they're pretty accomodating. I've only had to "throw down" once when someone said I shouldn't be in the road, and for that I just shouted back that Massachusetts General Law chapter 85, section 11B says that I have all the rights and responsibilites as drivers, plus I get to pass on the right. That elicited an "oh," and made me pretty happy.
It's not the optimal cycling environment (I've heard that Davis, CA gets that honor, and I know that Eugene and Corvallis, OR are better), but it's not bad, I must admit.