I would be cusrious as well. I'm about to purchase one myself, although I haven't bought it just yet.
I'm not really worried about the push/pull zoom. I've had a 50-200 push/pull zoom for 15 years, so I think I can handle it. Most people seem get used to it quite quickly.
Some are a bit concerned about dust. However a normal zoom lens with this level of deep zoom would probably have this problem as well. What I read on some other forums, there not too many people ACTUALLY reporting serious dust problems with the 100-400. And those who did have it, also had it with normal zooms in heavy dust like environments.
The weight issue was also a consideration for me. However, if you really want power-zoom or a prime 400mm tele, you'll just have to cope with it.
I'm still considering the 70-200L IS f2.8. However it is considerably more expensive, and combined with a 2x TC [140-400] it would also end up as f5.6 glass, just like the 100-400, but it would be even more heavier.
These are my options at the moment:
*combined with TC [Tele Converter]
price in Euro
2.8 all the way
1570 gr [non IS version 1310 gr]
*140-400 above with 2x TC
5.6 all the way
1835 gr [non IS version 1575 gr]
70-200L [no IS!]
98-280 above with 1.4 TC [won't handle 2x TC, maybe taped it will??]
420 above with 1.4 TC [won't handle 2x TC, maybe taped it will??]
If weight is all important, the 70-200L f4 would be a good choice, even with 1.4 TC.
I personally would use it a lot to shoot radio-controlled model airplanes, so most of the time I'll be using at the higher end of the zoom range [300-400]. Therefore I tend to go with the 100-400. I seriously considered starting with the 70-200f4, which is a nice lightweigth lens, but since I'll use a TC with it, I would like to have IS.
Still considering my options.....
PS. Here are some previous threads that may help you:
[Edited 2003-07-13 23:22:39]
Immigration officer: "What's the purpose of your visit to the USA?" Spotter: "Shooting airliners with my Canon!"