I have taken the Kenya Wildlife Safari tour with Go Ahead in Novemebr of 2007. It was a great trip, the trip of a lifetime! I chose to do the Tanzania part at the end so I was able to see the Wildebeast migration because it's always a good time to go if you go to Tanzania because the animals make a circle between the two countries. The weather was great and it was not too too hot.
If you decide to book the tour make sure you bring lots of extra memory cards or film, then animals are incredible. I would not have changed a thing about this trip, the people, the food, the culture was all the best in my life!