Ok, I'm in the same pedicament as many others. Which should be my first lens to use with a 20D? For me it's a question of the 17-85 IS USM or 24-70L USM at coming on for 2x more cost. But I could afford either - so budget, *if* I think it's worth it, is <£2K. Really I'm trying to work out best 'value' with value meaning what I get out of the camera - how much I end up using it, how impressed I am with the results, how much I *enjoy* it. I'm a hobbiest user not a professional. I take a big mixure of shots, inc portraits and some motor sport. Have tended to avoid landscapes if only because in the past (with my G3) I've been dissatified with the quality in the detail.
It goes like this, as far as I can tell.
17-85: Relatively slow lens (esp at 85mm), compensated for to some extent by 20D's great high ISO performance & the lens's IS. Half the price of the 24-70. Wider zoom range. Good 'one lens'/carry round solution. Comments about too much CA at wide angles, but I could always avoid using it below 24mm unless there was no alternative 
. Seeing bad CA after spending all that money will not seem like good value to me, I fear (a bit like seeing moire on a D70 setup). Smaller apertures will make getting narrow DOF harder when I want it. May lose it's 2nd hand value quite quickly, as a non-L, EF-S only lens.
24-70L: f2.8 throughout the range means shutter speeds should be fast esp. at high ISO. Thus the need for IS is less. Pretty much unquestioned glass quality. Will be easy to sell if I need to, even into the future. Narrower zoom range - ultimately more lens changing when I get more lenses. 2x the price. Bigger and heavier - no point in having better glass if it means I don't have the camera with me so much. Plan would be to get 'proper' prime wide and tele lens later.
Unfortunately 1.4 and 2.0 teleconvertors don't work with either lens, do they? 
Decisions decisions... I've so rar resisted picking up a mint 1D for ~£1500 as it's just too big and heavy (and really outside my budget by the time I put a lens on it), and I fear I'll always be "iffing and butting" as you whether to take it with me...
W
