I bought this lens to replace the EF 28-80mm USM V that came with my Elan II. This lens is superior in every respect.
* It has a REAL USM. Although the 28-80mm had a micro-USM, the ring-type USM found on this lens eclipses the other. It allows for full-time manual focusing, and the inner/rear focusing is fast and silent. Another added bonus is that the front lens element does not rotate like on the 28-80mm so you can use a polarizing filter easier.
* Better range. The 28-80mm was a wide-angle to medium telephoto. You had to get pretty close to your subject to get a nice portrait with the lens set at 80mm. The 28-105mm allows you to be at a more comfortable distance from your subject. It also allows you to utilize more of either the EX and EZ series flashes that zoom (my lens goes well with my 420EX).
* Better optics! The 28-80mm was a fine lens to learn with, but thats about it. The images it produced were so poor (even stopped down) that it almost makes you wonder what you're doing wrong! The 28-105mm blows its rival away. Although the performance isn't bad wide-open (aka using the max. apeture), when you stop down to f/8 or f/16, the results will really impress you. The colors are vibrant and the subject will look alomst 3D.
Bottom line: although there are, of course, the L-series lenses, this is the best consumer zoom Canon makes (only the 28-135mm USM IS would beat it, because of its Image Stabilizing technology).