Philip Wilson , Oct 20, 2008; 06:12 p.m.
Depends on the camera (Full frame or APS-C) and what you shoot. I shoot a lot of ski racing so I tried a friends 100-400 but was disappointed. The lens is slow to focus (especially when tracking) and does not give very fast shutter speeds. I was also not very impressed by it's edge of frame performance. Thus I bought the 70-200 F2.8 (non-IS to save money) and 1.4x and 2x convertors. the 2x convertor is not worth the money as it is a poor performer but the 1.4x is good on the 70-200. I think that indoor sports may also be a problem as i have a son who plays ice hockey and find F2.8 to be marginal as I can't get better than 1/125 of a second. In general for ice hockey I use an old canon FD with a 85mm F1.2 or 135mm F2 for this purpose (although I miss AF). i still shoot film and will only move to Digital when I get a 5D Mark II later this year. I suspect that the APS-C will help the edge performance (we have a Digital rebel that produces OK pictures with a very poor lens that is unacceptable on my 1Vs). Having an ability to work above 400ISO will also help the digital case (this is one of the reasons I am getting the new 5D). A friend of mine has the 1D Mark IIN and he has both the 100-400 and 70-200 and a rarely see him use the 100-400. I would also be careful of non-Canon lenses if you plan to keep the camera. they are not as compatible as the Canon lenses - for example my Sigma 14mm EOS lens will not work on digital cameras but works on all film EOS cameras whereas Cannon lenses of the same (pre-digital) vintage will work on all EOS cameras. The lens physically fits the Digital camera but the camera will not function.