Biking year round in Boston is definitely doable. I've been doing it for 8 years! The most important things in winter: lights, thin hat to go under your helmet and cover your ears, gloves, and light waterproof pants to pull over your regular pants for when it's raining or snowing. Wear one fewer (or lighter) layer than you would if you were driving or walking or else you'll get too hot. The only time I won't bike is if the road is full of snow. Motorists are actually surprisingly patient when there is snow on the ground. Sometimes you have to ride in the tire tracks of cars, but most people will just wait behind you. In the last few winters, I've seen more and more people bicycling than I have in the past. This makes it even less stressful since you're not the only one out there and other people actually expect to see you.