I find that I need a shutter speed of at least 1/1000, preferably higher, to get sharp images with the 400f/5.6 hand-held and it is not much better with a monopod.In my opinion, it is simply preposterous that Canon still makes a 400mm lens without IS. When I want a 400, I put the 1.4xTC on my 300mm f/2.8 II. It's expensive but not too heavy, and four stops of IS make all the difference in use. Or, I take the 100-400mm when weight and size are concerns or I need a zoom. What's the pint of carrying a lightweight lens if you have also to carry a tripod to get the best out of it?
I agree with all of this and find myself doing the exact same thing... I love my 300 II and my 100-400. Never liked the 400 5.6 when compared to these other two lenses.