I was here looking for options before, and a bunch of places are closed. I highly recommend going to Back in Action bikes if you race or are fairly serious about your biking. Zach is one of the only people I trust to work on my bike. Having also seen him interact with the other customers, he treats someone's beloved commuter as seriously as my "nice" bike and will give you all the ins and outs of the repair if you ask.

Other good options are Ace Wheelworks in Cambridge if you're over there.