Very Good Hotel in a Very Good Location
As you consider this review I need to let you know that I spent the first 45 years of my life living in the Boston area until moving to Phoenix 7 years ago; so I am very knowledgable about Boston and had some specific interests when choosing to stay at the Onyx. Also have become a fan of the Kimpton Hotels and their style from multiple stays at Kimptons in San Francisco and New York.
So, with all of that said, would highly recommend the Onyx in Boston for a business or pleasure trip. It is right next to the Boston Garden re: Bruins, Celtics, concerts and events; close to numerous pubs/restaurants (The Fours, McGanns, The Harp, Porter's, Sulilvan's Tap etc...) and a two minute walk to the North Station T/Subway station (green line) and 10 minutes to Faneuil Hall. However, it is in an office/industrial neighborhood so if you are looking for green grass, trees and expansive open space, this is not that type of location. The hotel is well kept, clean with a great staff. If you are a member of the Kimpton loyalty program you will get free continental breakfast each day. The rooms are adequate in terms of size, bathrooms good and the rates at the hotel are generally reasonable, especially when compared to some other downtown hotels. 
If you are looking for a solid hotel at a good rate and need/want easy access to the North End, financial district and Boston Garden area I would recommend the Onyx highly.