This was the last wide-angle lens I tried on my 20D before ultimately ending up with the 5D and the excellent 35mm f/1.4L (35mm is like 22mm on a 20D).

Compared to the 17-40mm f/4L, the 16-35mm f/2.8L is sharper, has much better colors, and focuses more reliably (f/2.8 lenses enable usage of the 20D's center point cross-hair AF sensor). The lens focuses well at infinity and is very good for colorful landscape shots.

However, for $1370, I expected more. It was very sharp on the 35mm end, but softer than I wanted on the 16mm end. I really wanted to love the lens. In fact, I tried two copies, which ended up being about the same.

The 16-35mm is soft in the corners on full-frame particularly at 16mm, but Canon came out with a new 16-35mm f/2.8L II lens that is sharper edge-to-edge at 16mm, though the MTF charts don't show any improvement at 35mm. Canon also announced a new 14mm f/2.8L II ultra-wide prime in August 2007 and it looks very promising for shooting architectural interiors, etc.

On a 1.6x crop body, I'd defiinitely get the EF-S 17-55mm f/2.8 IS because it is very sharp and because it has more reach and IS. I never had a chance to try it while I still had the 20D.

I recommend the The-Digital-Picture.com for comprehensive reviews.