This is supposedly Canon's best zoom lens. It's certainly very impressive in terms of price ($1700), size, and weight (3.2 lbs). People will stare and wonder what you are trying to prove (or compensate for :-).
The lens has a solid build and performs very well in terms of sharpness and contrast. I had it bolted onto my 10D and 20D for about a year and a half. I didn't mind the weight at all. But I didn't feel that the image quality was excellent enough to justify the price. Sharpness at f/2.8 was disappointing. The lens didn't focus reliabily enough on my 20D. I find the colors from the lens rather "blah" and flesh tones somewhat "pasty" for lack of a better word. The lens didn't take the TC 1.4x as well as, say, the 135/2L. I never tried the lens on my 5D, though I doubt it would affect the result much, particularly regarding the colors.
Unless you really need a zoom in this range, I believe a set of primes can deliver a MUCH better result, even with the 1.4x TC. For less or not much more money, you can have much better image quality (sharpness, contrast, colors), faster and more reliable focusing, lower-light usability for action shots, more sharpness wide open, more bokeh, and less weight to lug around. It was a no-brainer for me. Here are some choices for various budgets:
$1530: 85/1.8, 135/2L, 1.4x TC - this is what I tried first with my 20D and then 5D - excellent combo
$2350: 85/1.2L II, 200/2.8L II - this is what I have now for my 5D (plus the 300/4L IS) - even better, there's really no substitute for the amazing 85/1.2L
A zoom alternative is the non-IS version of the 70-200 f/2.8L ($1140). Some say it's sharper and focuses more reliably. Others say the IS is key and the IS version is sharper in some cases. Many lately are saying that the newer 70-200 f/4L IS ($1060) is the sharpest of the bunch, it has a newer version of IS that can provide up to 4 f-stops, and it's a lot lighter (1.7 lbs). It's a moot issue for me now that I'm committed to primes, though I'd like to try the 70-200 f/4L IS.
I recommend the The-Digital-Picture.com
for comprehensive reviews.