To me September in Boston is one of the best months for riding (if not the best) - the days are getting shorter but the weather can be nearly perfect. October starts getting rough as things start cooling off and it is completely dark after work. As others have said if you have some decent lights on your bike you'll be fine in the dark.

I don't think I will ever be a winter rider though - it just isn't fun riding around in freezing temperatures no matter how bundled up you are (to me). Luckily I have an easy bus ride into work as an alternative. Freezing temps and bike lanes full of plowed snow are really tough.