Nairobi is no more dangerous (in my opinion) than large cities anywhere, provided that you apply common sense and basic caution. You shouldn't walk around at night, and there are certain areas to avoid. You shouldn't flash your wallet or cellphone around to attract attention in public places. (However, this is true in Detroit and areas of LA and New York!)
I'm here with my wife and daughter, and I think it's reasonably safe, as long as you're careful and aware.
My opinion, at least.
I've been offered a job in Nairobi. Many here tell me that it is not a safe country to live. Specially after sun set. I'm to work as a business manager. If some one could advice me on the ground realities of Kenya, about the safety I would be thankful. I would travelling with my wife and my 1 year old daughter.