Yup, I went from a S3IS to a 300D a while back and couldn't be happier. Well, I do miss live view, video recording, and focusing manually is harder. The price was right though: I sold my S3 for $175 and got my 300D for $180 the next day.
The big difference in the two is sensor size (12x bigger IIRC) and all that goes with it: ISO400 on the 300D has about the same noise as ISO100 on the S3, and the 300D can go up to ISO3200 if need be. Shots that I needed a tripod for, can now be hand held. Exposures that took 5 minutes now take 30 seconds. I I put it on ISO100 then things are silky smooth.
The range of lenses is a nice help too: Reversal rings, and extension tubes give great macro results for cheap. Another cheap option I like are 50mm, fast lenses, they are great if I don't want to use a tripod: Even a F2 lets in about 2.5x more light then the S3 at F3.5, I just wish I didn't have to deal with the crop factor that are the bane of APS-C sensors.