Not great, not terrible
I spent only one night at the Onyx in July 2012 while traveling to Boston on business. If I were returning to Boston, I would choose another hotel (particularly if I am paying the bill). In my opinion, he Onyx is priced too high for what it is.
The black, white, and red color scheme is fun. So are touches like a leopard print bath robe (though it looked like it had seen better days -- time to replace it). But the rooms are tiny. Very, very small rooms. Its an old property that was refurbished obviously by Kimpton but it feels old and small. I also felt the cleanliness of my room wasn't great - the carpet looked stained or un-vacuumed, or both.
The location was perfect if you are going to a basketball game or concert at the TD Garden -- just a block or two away. But otherwise, not that fantastic of a location. Along a dark, narrow side street, there are many bars though very close by. It seems like kind of a low-end area, many sports betting places and cheap bars.
The front desk staff was friendly. I didn't try the restaurant/bar so I can't comment on that. I found a bottle of red wine and chocolates waiting for me in my room on arrival -- but with a card addressed to another person which seemed disorganized (and as such I didn't touch it).
Overall, I wasn't displeased, but I wasn't paying the bill.