This is my first 'L' lens and I am very impressed with the quality, both physically and in the images it produces. It focuses a lot faster than my cheaper lenses, and feels about the right weight. The bokeh is smooth, and I am pleased with the high percentage of great photos straight from the camera.
It isn't a cheap lens, but when used on a 1.6x crop body, like the 350d it becomes a f/2.8 320mm - great value for money!
The only problem with buying it is that you will want to get more and more L series f/2.8 lenses!