Overall, very good but for ending. I was tired when I read it (so mileage may vary), but I did not enjoy the unsatisfying wrapup. It is a very well written book and a fun adventure to follow, so I recommend it. And I strongly recommend reading the extras in the back to gain a bit of background on the setting. I didn't do this on my first reading and my lack of enjoyment of the ending may be the result.
I will also add that it is not always easy to tell who the 'good guys' are in this story, if this is your first Culture novel. That can be frustrating but is ultimately rewarding. Keep your mind open about the Culture. Real life is not 'black and white' either.