I have had this lens for nearly three years and shot thousands of images, first on a 350d and now on my 30D, most with this lense. Though not all were taken with this lens, many of my images, hundreds?, have been published in local newsletters, been included in calendars, including covers, and sixty or seventy images hang framed on peoples walls. I've been in shows and had a couple of one-person shows. Landscapes, historic places, portraits, group shots, wildlife, and flowers, wild and garden, not to mention family and friends. This lens is on my camera at least 70% of the time.
I would mention that I have a beautiful 17" printer, and so blow my pics up, full-frame and cropped. At that size--most often 17"x22" and sometimes larger, you see what an 8 meg camera can do and you see what a good job this lense can do. I have two L lenses and have owned two others and while this lens is not an L, it can provide excellent image quality. Leaving speed aside, I can "get" ninety percent of the images I am trying to get.
When I go light day hiking, as I often do, sometimes in fairly steep or rugged terrain, and only want to carry one lens, this is it. It has been used outdoors a lot, and I do mean a lot. And in some pretty severe conditions too. And it works as well as the day I bought it three years ago. No lens droop like the 28-135, which was my walkaround lens on my film cameras.
Given its range, on Canon 1.6 crop cameras(Ds and ds), there is no other comparable walkaround lens, no matter the cost. There are lenses that have better image quality, but not by as much as you'd expect, and there are faster lenses, but given the image quality at iso800 and even higher of every current and recent Canon digital body, that is less of an issue than it once was.
The lens has three issues: chromatic aberation, distortion, and lower-light image quality. Chromatic aberation: I'd guess that 10% of all my shots have either red or more rarely cyan CA, mostly back-lit scenes and angled sunsets with sharp light on one side and darkness on the other. I only shoot RAW so I can get rid of 90% of this in PS Raw Processing in one quick step. Some images take more work and are impossible to perfect, but I have not had one unuseable image due to CR. Distortion: There is some at the wide end, but, if you pay attention when you shoot, you can either have it in your image intentionally or mostly not have distortion. I never correct for distortion, though I sometimes crop. I have not had one image that I have had to throw out because of distortion. Finally, image quality: compared to an L lens, this lens gives you 85% of the color/contrast/perceived-sharpness in low-light shooting. (I'd say 90% in better lighting).
In conclusion: Even for less than a thousand dollars, it simply cannot be beaten in terms of range. For the non-professional using a Canon crop camera, I believe it is the perfect lense. For the professional, it is imperfect, but a workhorse if you forget that its not an L and if you are willing to do modest post-processing.