Ex-boston resident here. You are allowed to bike in the streets, pretty much everywhere. You can bike in bike lanes, and you may take the full lane. I may not like it, because there always seems to be a bike in front of me going 15 mph less than I want to during heavy traffic, but it's completely legal. And fairly safe.
You are also allowed to ride on the sidewalk in many places in boston; you choose when that's smart.
I would not bike during snowstorms or soon after; I've had a bike wipe out right in front of me crossing the tracks (which are of course incredibly slippery). Someone being a little less careful, or without good tires or ABS, might end you - a silly way to die, at 10 mph.
Just don't, for fucks sake, bike the wrong way down one-way streets, around blind turns, at 20 mph, in the middle of the road. There are a lot of roads like this. One way signs apply to bikes, too. That was the only time I came close to killing a guy in Boston.