This is located in an awesome location - the South End. Walking distance from the Back Bay station, easy access to the orange line which gets you to some of the best spots in the city, and walking distance from some of the best food/bars IMO. I used to live in Boston, and this is just about a 10-15min walk from my favorite Spanish restaurant in the city: Toro; only 5 min walk to Anchovies for late-night drinks; 3 min walk to Petit Robert Bistro for a delicious French brunch.
While I can't speak to whether this is great for long-term stays, since we just did a 2-night stay, I will say it was perfect for our basic needs after a long day of being out and about. It's hard to find BnBs in this price range with AC in Boston, but this place has it so you'll stay cool during those hot months.
One thing I will point out, this is in a very old building - a Bostonian brownstone. The walls are very thin and we were able to hear the neighbor next door quite loudly. Not a deal-breaker (especially if you're not planning on being in the space too much) but something to keep in mind. Other than that, it was great for our purposes. Highly recommend for traveling singles or couples!