I'm a regular Serena guest. I must say though that I do prefer the Kampala Serena. The Nairobi Serena is simply just showing its age, I suppose. Its a bit darker, the furniture older, etc. But still perfectly lovely. Centrally located. Overall good and efficient service, decent-to-good wireless internet access throughout the hotel. Good selection of satellite TV options.
Breakfast buffet is good, room service is okay (wish for a larger menu -- but I say that about all hotels).
Avoid copy/printing business travelers, like most Africa hotels, crazy expensive prices.
I do like the gift shop -- actually they have two, one with toiletries, phone cards, snacks, postcards/stamps, etc. The other with jewelry, baskets, and other local crafts/arts (priced high of course, but no worse than the airport and much higher quality than you'll find there). You can change currency to Kenya shillings here, but cannot change Shillings back to USD/Euro/etc. There is an ATM right next to the hotel, less than a block away which is convenient.
I've now spent a total of 3 weeks in 2012 and 2013 here and overall I have been satisfied, though this time I had a terrible AC unit in my room that simply did not cool the room enough during the day.