Fantastic location! A few things to keep in mind about Billy's:
The pros: the location is EXCELLENT. the apartment is just off the Boston Common and about one short block from the subway.
Directly below the apartments on the ground floor is a pizza place which is very convenient if you're feeling lazy, and if you want to stretch your legs even a bit you're only minutes from pretty much anything you could imagine.
The cons: The apartment is quite a bit smaller than the pictures indicate, although it's adequate for two people - as long as they don't mind squeezing into a double bed.
It was very clean but very outdated and the bathroom in particular needs some remodeling. It's also on the sixth floor with no elevator, so keep that in mind if you have luggage - certainly was a nice way to get some exercise.
The door to the apartment was pretty flimsy and appeared to have been kicked in at some point in the past. We only had the key to the doorknob, not the deadbolt. Luckily that was enough, although it didn't inspire much confidence leaving our valuables.
Overall Billy was very easy to communicate with and we stayed during a very busy summer weekend, so the price is understandable if a bit steep for what you get.