Chris, I have a friend who is a driver/guide and has his own 4x4pop-up roof minivan.
Ben is reliable and good company. He also has good birding knowledge and knows many of the local birders -useful for tracking down endemics and range-restricted specialties.
Do you have an itinerary in mind?
Two years ago we went to Mount Kenya, Samburu, Baringo, Nakuru and the Mara. Last year we went to the Aberdares, Baringo/Bogoria, Kakamega, the Mara and Naivasha. Both trips greatly exceeded all expectations.
Kenya is expensive however due to accommodation costs and National Park fees (the Mara is $80 per person per day for non-residents!). It is better if you can share costs.
I get Bernard of Liontrails (
http://www.liontrailssafaris.com/ ) to arrange my itinerary/book accommodation. He is excellent to deal with and keeps overheads as low as possible.
Here's a report of our experience at Baringo/Bogoria in 2010/2011:
http://www.birdforum.net/showthread.php?t=229458
Our experience of the Mara migration in 2010 was also amazing. Even though these were family holidays I managed to get some reasonable images - mainly due to the skill and knowledge of Ben and his local contacts.
I'm off to Kenya again in a week's time. Doing an eastern circuit taking in Amboseli, Tsavo West, Shimba Hills, Watamu area (Arabuko-Sokoke Forest) then back to Nairobi via Tsavo East.
For general wildlife viewing combined with spectacular birding I don't think there is any better place on the planet, but you do need to plan a decent itinerary in advance and then organise the trip through people who know what they're doing.
Steve