It is my most used lens. Once I bought this one, I sold my 70-200 f/2.8 because it never got out of the bag. Even though the IS is the old 2 stop version, it's very effective. Apparently, when the lens first came out, there were some soft copies, but I think Canon has fixed the problem. I know mine is very sharp and I think all of the ones made in the last few years are. You'll probably hear some people complain about it being a push-pull lens too, but I found that I really like that feature and it didn't take any time to adjust to it.
You'll probably hear some complaints about it needing a lot of light too, so here are a couple of images shot handheld in low light to give you an idea of how it functions.(oops - Tom must have been posting the same time I was writing this)
This was taken before sunrise with a 30D.
1/30s f/5.6 at 400.0mm iso400
This was taken in the shade of a large tree (notice the flea on his chin - gives you an idea about how sharp it is)
1/80s f/5.6 at 400.0mm iso500
BTW, I also had a Canon 400mm f/5.6 L lens and the zoom was just as sharp and much more versatile. I returned the 400 after comparing it to the 100-400.