I spent just one night at the Mennnonite Guest House as I flew in late from Europe and had a meeting in Westlands the next morning. I was really impressed with the place - my booking was handled efficiently by email, the key was left as promised with the watchman, the room was simple but spotless and everything worked. The breakfast was friendly and guests are expected to share tables and chat, which was a pleasure as the place was full of people doing interesting things.
The gardens are beautiful and a tranquil oasis amidst the development of high-rise apartments and office parks in this part of Nairobi.
People should note that the guest house is really designed for missionaries and development workers, and the low prices shouldn't be mis-used by travellers on business trips or safaris that don't fit with that ethos. It's not a regular hotel, and standards of service therefore shouldn't be compared directly.