While I could easily sugarcoat my review and say this lens is the greatest thing since sliced bread, it wouldn't be fair to those seeking an honest evaluation. Instead, I'd like to point out its strengths and weaknesses based on usage with the Canon 20D:
1) WIDE OPEN (24-70mm). Wide open (F4) performance is great at 24-70mm. At this range, the lens really excels in sharpness and detail. I would be so bold as to say the sharpness of the 24-105 wide open at F4 rivals my 35 F1.4L stopped down to F4 - it's that good. The sweet spot seems to be between 35-55mm (perhaps with sharpness peaking at 50mm). Also, corner sharpness on the 20D (as expected) is very good.
2) RANGE and IMAGE STABILIZATION. On a FF camera, the range would be perfect, though it's still very useful on a 20D if you have a wider lens to supplement it. I have the Tokina 12-24 F4 to go along with this, and I think they will be a great combination. That said, the range at 1.6x will be sufficient for 90% of the shots I take. The IS is no surprise really, it works as advertised and will come in handy quite often.... though not for shots of my kids! ;-)
3) CA/PF. Surprisingly, CA/PF seems to be VERY rare - and I have looked for it in all of my shots. You won't see any of that nasty stuff you see on some lenses with wider apertures (my 35L has some heavy CA at F1.4-2 for example). I would not be afraid of using this lens wide open at any time because of concern for CA.
4) BUILD QUALITY. Build quality is excellent, as you would expect from an "L" at this price point. The zoom is just right - not too stiff (it was a little stiff at first), not too loose to cause zoom creep. The lens is hefty, but not too heavy for a "walkaround" lens. I'm hoping to use this lens for years to come, and there is no indication that it won't last for years. The lens comes with a hood and pouch. I'm also using a multicoated UV filter for protection.
5) COLOR/CONTRAST. Obviously this is something that is subjective and can always be corrected in post-processing, but this lens gives some very good results straight out of the camera. Though I don't always agree that the difference is major, I have to admit that the "L" lenses do have an edge here.
1) WIDE OPEN (80-105mm). I have noticed some softeness with this lens wide open at longer focal lengths (105mm in particular). It's not unusable by any means, but definitely not as sharp as the shorter range. Stopping down the lens improves the sharpenss a great deal, however.
2) DISTORTION. As many other reviews have noted, there is some barrel and pincushion distortion. This may be a concern for those who shoot architecture or subjects with important lines, but it will not be a concern for most shots. I have not done any tests to measure the distortion, but I thought I'd mention it because it seems to be a bit more than the 24-70 F2.8.
3) PRICE. $1100-1250 is a lot of money for a lens, and I would hope the price comes down a bit after us "early-adopters" snatch up the early shipments. However, with a contrant F4, a decent zoom range, IS, and L quality, its worth seems to be significant.
OVERALL: With a great range, excellent build, useful IS, and exceptional optics (especially in the 24-70 range), this lens is a real winner. It will certainly be used on my 20D more often than not.