This is first lens upgrade ever. I jumped from at 18-55 kit lens to this. I have a Rebel XT and was looking to get a new lens as I have definitely grown beyond the kit lens.
Was it worth it? Well, after taking this lens out for an entire day yesterday, I can say it is worth every penny. The image quality on my copy is very high. Without any post processing, it is SHARP wide open and even sharper when stopped down slightly. Who said quality only comes on L glass? Well technically, the glass inside is L glass, but the build on the outside does not live up to L lenses that I have seen. The focus ring and zoom ring are nice and smooth.
The IS is a very nice feature. I did some tests and it really does save you in low light situations. combined with f/2.8, i can take some 0.5 second pictures without any noticeable blur. for pixel peepers, you will be amazed. I think this features pays for itself in the end. it's the difference between a ruined shot and a saved one. I just leave it on all the time and have not noticed an increase in battery usage. I took over 400 shots yesterday (with the extra battery and grip) and still had enough juice to download the images to my camera.
The bokeh aka background blur wide open is so smooth. very high quality saturation and colors as well. during post processing, i had to adjust very little (if any) in curves, sharpness and levels. For the non-believers, this really is a high image quality lens. Although I only have a Rebel XT, i'm sure 20D and 30D users will experience similar results.
The price is totally debateable. If you are looking for something in that focal range, definitely check it out. I thought IS would only be good for long range, but it definitely helps out here too. it's a pricey lens, but if you know how to use it, you will come up with some spectacular results.