This is my first L lens and it has exceeded my expectations. Before buying it, I worried for a long time that the difference in image quality wouldn't be obvious to me, or that it wouldn't be worth the money even if it was obvious. Having bought it, I have no regrets -- the quality is breathtaking (and I am using it on a humble 450D).
The results are superb. In the shop I compared it with the Canon 70-300 f/4-5.6 IS, and the difference in image quality was overwhelming even when viewed on the modest LCD of a 450D.
It is easy to worry obsessively about sharpness -- no need with this lens, it is wonderfully sharp. Where it really shines, however, is in the colours it reproduces. Saturation is really good. If you, like me, have never tried a professional lens before, you really will notice the difference. The IS is very useful -- it makes low-light shots at 1/15 easily possible, provided your subject is still. It certainly gives you at least a 3-stop shutter-speed advantage if you have reasonably steady hands. I have not noticed any chromatic aberration, even in circumstances that would normally be very harsh for a lens.
I don't really have any criticisms. It is a beautiful lens and it helps to produce beautiful photos. It isn't noticeably heavy (at least to me) -- the f/2.8 versions are a lot more bulky. Of course you sacrifice something by choosing f/4 over f/2.8, but the bokeh on this lens is lovely nonetheless. It is a bit long on a crop-sensor body to be a general walkaround lens, but it really shines for portraits (head-shots particularly).
I wouldn't hesitate to recommend it. In my opinion, it is easily worth the money.