Clearly IS is useful for you and many other people, but not much for my style. A monopod or tripod when needed is enough for me, and I can leave that extra weight behind when not needed, whereas the extra weight of IS (and f/2. is always there.
I imagine I'd feel the same way, if I didn't have the narrower FOV that the 20D gives me. That is, I certainly can take sharp images with a tripod at 200mm, but the number of keepers are lower, because any miniscule movement shows extremely easily. I also like to be able to take a hand-held shot of something in those cases where I don't have the time to whip out my tripod. It is
a heavy load, and if I'd been backpacking like didger, I would
think twice about bringing it along. I might even buy an f/4L as a backpacking option.
I also do some indoor photography, mainly of pool billiards, and that's something I simply must
have IS for. Even at ISO 1600 and as open as f/5.6, I'm at between 1/15 and 1/40 second. (I'd really want f/11 or so for better DOF, but the lighting just doesn't allow for that.) Tripods are rarely possible to use in a non-interfering manner.
And since you are using a 20D, why is EF-S vs EF an issue, apart from the likelihood that a competently designed lens optimized for the format and sensor type in use is likely to perform better than a lens designed for some other format, and maybe for a different sensor type (film)?
For me, this isn't an issue in itself, except that I can't put it on my 650, which I hardly use anyway, and that I can't put it on a 1D series camera, should I ever upgrade to one.
It could be an issue when considering selling the lens, but I don't think so; the vast majority of Canon DSLR owners will most likely have an EF-S compatible camera.
But the EF-S lenses do
restrict your choice of camera
more than a regular EF lens.
I know you're aware of this, but it looks like it didn't come across that I might be thinking about it, sorry.