There are now four cellphone service providers in Kenya: Safaricom, Zain, Orange, and the newest of the pack, Yu. Safaricom, Orange and Zain all provide unlocked cell phones (which are also available in big department and electronics stores), mobile phone services (SIM cards and prepaid minutes, or postpaid plans), and mobile modems for internet access.
Safaricom was the first provider, and so has the largest installed base of users. People who stay with them say they do so because their friends are on Safaricom, and so it's cheaper to call to users on the same network.
As it’s probably the most expensive network, and as people start texting more, this may not continue to be true in the future for current users.
Speed and reliability
I have tried Safaricom, and Orange and a friend used Zain, including the mobile modems; plug-and-play gadgets that plug in to the USB ports. I find the EVDO mobile modems by Orange by far the fastest. Recommendations from friends confirm to me that Orange at this time is the fastest, most reliable network. For reasons not entirely clear, the Safaricom network seems to go down the most often. Perhaps this is due to frequent overloading.
I had trouble with installing both the Safaricom and the Orange modems. The first (Safaricom) because the instructions were not provided, the salesman neglected to help me in the store, and the on-line instructions (accessed through my Blackberry) were hard to find, incomplete and confusing. The second (Orange) had instructions included, but the store (not an Orange store, but an Orange distributor) gave me the wrong SIM card for the modem service I tried to sign up for. When I contacted Orange, I ended up getting personal service over a period of several days from a technical support person in Nairobi. In the end, however, the technical support person in the store figured out the problem as soon as I went into the Orange store and he saw the SIM card. The Zain had a little trouble that settled after rebooting.
Service and attitude of providers
My greatest interactions have been with Safaricom and Orange. Safaricom employees have been, in general, almost uniformly rude and unhelpful, even when I bought 10 phones at once! The person who sold me the phones did not activate them, despite my request that they do so, giving me about an evening of wasted time while some Kenyan friends and I figured out how to activate them, which was not intuitively obvious, nor were the instructions accurate. I believe that the poor customer service attitude is because the stores are jammed almost all the time, and they don't seem to care about customers, as they have too many. Thus their customer service is, at least in my experience of it, terrible. Orange personnel, on the other hand, have been uniformly helpful and friendly, and go out of their way to solve your problems. I have heard from friends that the Zain personnel are also helpful.
If you're in Kenya for any length of time and need to interact by phone with Kenyans, it's more affordable to buy a cheap unlocked cell phone (if you don't have one already), a SIM card and airtime minutes, then use a global service provider (such as Verizon).
To gain internet access, I suggest a mobile modem – they work pretty well. Make sure you convert your minutes to data or you will go through the airtime remarkably quickly.