Though it is a far cry from luxury, the ACK Guest House Nairobi is located in a very convenient location and serves delicious food (especially breakfast). As others have stated, the hotel is totally safe and solid.
The guest rooms (especially the ones located in the behind the main building) are spacious and have all of the necessities. Since this is a religiously affiliated hotel, they do not serve alcohol. If you are in the mood for a drink with your dinner, you can take a very short walk down the road to the Fairview for dinner and drinks (sit by the pool if it's warm enough).
I've stayed at ACK several times in the past few years and always have an enjoyable experience.