This thread was quite a help to me thanks to everyone. Im in CPT now after 7 days on walking and riding safaris in Zambia. Shot about 4000 shots. Used the Canon 100-400 f/4.5-5.6 L IS lens almost exclusively. I would not have wanted a 16 pound lens while walking about in the mud in the Zambian weather. But the lighter one did great and for most things there was enough light that I was shooting at over f11 anyways. The night shots were a different story but f2.8 doesnt help a lot when you are shooting at something in a spotlight. Almost 10K exposures taken in the last year with the 20D and I do love it.