I have been using the 20-35L 2.8 for about 4 months now and I don't think I would go without it. On my EOS 3, it was a superb wide angle, showing very little distortion on the edges considering its width. On the 10D, it is equivalent to about 35-55 which is not ideal wide, but still a good reach.
On the 20mm end, you have to stop up to around 5.6-8 to get the optimal sharpness and contrast. However, when at 30-35mm, the 2.8-4 is more than sharp enough.
The only issues I've had with the lens thus far:
- it is prone to flare. But show me a WA that isn't. Compared to some of the 12mm zoom offerings from Sigma, flare is not a problem with the 20-35.
- Chromatic abberation can be marked in some instances, most specifically around tree branches surrounded by light sky. This is not unusual for most lenses, but it is rather substantial for an L lens.
I have yet to use a 20 or 24 prime. I have always thought of picking one up, but then I start to wonder what the point is when I have such a nice lens with the zoom convenience.