For wildlife photography, I have been generally happy with a 420mm f/5.6 fixed lens system (300mm f/4.0 L IS USM + 1.4x converter) with a monopod. On the 60D it is light enough to carry on long treks in the field. I use a monopod. The past two weeks while traveling I have been using a borrowed Canon 100-400mm l/4.0 IS USM zoom. It is MUCH heavier, but it has the great advantage of being able to shoot large subjects such as herons and alligators at close range without cropping. Worse, I did not have my monopod. It took many great photos, although I had trouble getting sharp focus on small subjects such as songbirds at 20-30 feet range. Using center point focus, it seems that the focus area is larger or more diffuse than with my 420mm system, so I had many bad out-of-focus outtakes in these circumstances. It seemed to focus on the foreground or background, or intervening vegetation that would not have been a problem with the 300mm_1.4 extender. It also seems not to focus as closely in the macro setting. BTW, I always carry a point-and-shoot for landscape and macro shots when needed. You may see my more recent shots and compare them with past ones on my FLICKR pages.
