I stayed at the Tribe Hotel for two weeks in October 2010. I was visiting Kenya for business, and after long work days, the Tribe was a wonderful environment to come back to.
The minimalist and trendy decor, warmed up with African art, friendly and always smiling staff, heavenly comfortable rooms/beds, free internet access, good food and a beautiful pool area, were all key factors in making my stay a memorable one. Also, the Village Market, a posh and secure shopping mall, which is located only a few steps away from the hotel, made for very efficient last minute shopping before I headed back home to Canada.
This hotel is not located in the downtown area, but this was not a key point for me, as I was driving to various remote areas almost everyday.
In a nutshell, I would strongly advise you consider this hotel, should you want a peaceful and comfortable stay if you ever visit Nairobi!