IS, is not essential, but if you encounter the atrocious weather as I did on my last visit, then ii would be a great help. When your driver comes across anything interesting he will stop for you to take photos.
If you're going to such as the Masai, many animals will be so close you could stroke them, so be prepared to swap lenses. The lions and also the rhinos where just several feet away, for the rhinos I used a 5D + 70-200. I was on foot and approx 10 feet away.
A 100-400mm is normally ideal, but on occassions it can be to long. I always use spot metering and servo focussing, but be prepared to switch to manual focus when the animals are in long grass. I have always taken two bodies, one with a 70-200mm and the other with the 100-400mm.
As you will be constantly changing position on the safari vehical, such as a tripod or other such aids are rather pointless. However a tripod does come in handy when at the base camp.
You can see a few photos from the masai, but the conditions where atrocious with constant rain and flooding everywhere, thus spoiling the photographic aspect of my last visit.
http://www.photoshop.com/users/hssut...b?wf=slideshow
Not all the photos in the gallery where taken on the Masai
Harry