Pricey (for what it is) hotel for flamboyant guests who don't mind paying.
The NineZero seems to have a faithful following, satisfied with this particular kind of “boutique” and prepared to pay for it. I have the feeling that the hotel are more willing to listen to fan appreciation than criticism offering another perspective.
The hotel is very well-located for my purpose of attending meetings on the next block. I think it is for this reason that my host uses this hotel for visitors. The location right by Boston Common is very central, but by no means unique. There are quite a number of hotels in this neighbourhood.
I arrived quite late and the check-in was efficient, with the offer of a complimentary glass of wine. The reception was friendly and the option of signing up to “free” Internet was alerted to. In fact, with one exception, the staff at the reception are all very friendly in a professional way.
The room was small but quite adequate. The desk was large enough for working and well-lit with a comfortable chair. The design and furnishing was, to put it understatedly, rather flamboyant. The room temperature was adjustable and functioned well. Bathroom was quite standard and clean. 
Breakfast was not included in the normal (corporate) room rate. It was à la carte and the food was OK. Service was rushed and certainly not worth the tip that was expected. The heating was set very high.
There was no electric outlet for the standard European two-pin plug. The reception had difficulty at first to find an adaptor. The surprise however was that they wanted to charge a deposit to the room. Charging $40 deposit for a $1-2 adaptor was simply ridiculous. The hotel also charged a $100 early departure fee (+ state, local and city taxes on top of that) for departure notified more than 24 hours in advance when the room was based on a corporate rate with no reduction for longer stays at the same time as the hotel is fully-booked, which my colleague was told when requesting a late departure. Of course one was too busy to go through the bother of looking at the fine prints and challenging whether there was any legal basis for this extraordinary fee, so one just had to pay it and leave. In doing this the hotel did a good job in making sure that I would never return.