I had one of these and found it wasn't as sharp as any of my other lenses, which included a 17-85 IS at the time. I've since sold both, but like all these things, you only really notice a difference when you do a direct comparison with something better (and usually much more expensive!).
If you believe what you read over on DPReview, Canon lenses can be variable in quality, so even if you spend loads on an L lens you may not be guaranteed good results, although that has not been my experience. I have noticed improvements in sharpness, contrast and colour, so I think they represent a good investment.