First up, this is a very heavy lens! I know this is typical for most white-barrel lenses, but personally I'm not used to it yet. Carrying the lens on left hand while holding 400D with the right earns me a painful wrist in 10 minutes or so. Maybe 5 minutes if a Speedlite is mounted as well.
(See my 17-40mm L user report for a summarised intro)
When in normal operation, I find this lens focal range a little short, because I'm used to the 300mm max focal length of my own lens. However, I must say the quality totally makes up for that.
Being f2.8, it is great for concerts with low light. I didn't really test DOF so I am not too sure about the bokeh capability.
Although a quick check at Amazon.com via the PG banners said that the f4 version was around 3 times cheaper, it might be wise to invest in the lens that works better in low light i.e. wider aperture.
I didnt need IS because I found many people can hand-hold telephoto lenses steadily at up to 200mm, but of course with not too long a shutter speed; the lens truly is heavy.
Because of its sheer weight, it takes some time and nerve to mount the lens(via tripod mount) on the tripod. Also requires a steady tripod otherwise the whole darn thing is gonna collapse. Highly recommend users stay away from ballheads - the risk of smashing your lens on the tripod legs is not worth it.
Build-quality wise, its again, rigid metal, expected of an L lens. This adds to the weight but I really think its good; you never know when your lens will be knocked around.
Also, the zoom and focus rings are 'average' of the L lenses I have used.
Optical quality, I did not really test because of limited use but I will update this thread as needed.