After reading several online reviews of the Canon EF 75-300mm f/4-5.6 III USM lens, I had nearly talked myself out of even looking at one. However, thanks to a local Canon demonstration, I was able to play with several lenses, the 75-300mm f/4-5.6 among them. I got to try it out alongside the IS version (which costs just under 3x as much), the non-USM version, and some of their L-series professional lenses.
That said, I found there to be less difference among the directly-comparable lenses (the non-USM, USM, and IS versions) than I'd have thought. On the test shots I took using a Canon Digital Rebel XT, I didn't find full-zoom telephoto shots to be appreciably softer in the non-IS version reviewed herein, nor were the images overly soft for my liking period.
The USM focusing didn't seem to make as much of a difference as I'd expected over the non-USM model, either. Focusing was still relatively slow (as other reviewers have pointed out), although once an initial focus has been made, adjustments aren't too slow unless changing to a subject substantially nearer or farther away. HOWEVER, the AF engine did make a number of "mistakes" when using this lens that it did not using the IS lens (or, of course, the L-series glass); more than once I had to either switch to manual focus or try multiple times to get the right focus "lock." Furthermore, the USM model doesn't get you internal focus, either, like with higher-end lenses, so the end still rotates during focusing, which can be problematic with a circular polarizer or other filters.
Overall, I'd say that you "get what you pay for"; this is a very inexpensive lens, and it shows in some areas. But it's not nearly as bad as some would make it out to be. I tried it out both on several indoors shots under less-than-ideal lighting conditions and was pleased in most respects, and outdoors, I got very good results on even moving subjects.
Pros:
-- Cost; at under $200, you would be hard-pressed to find a lens with the same reach with even half-decent optics.
Cons:
-- Slow focus
-- Autofocus seems to confuse somewhat easily over the IS model
-- The USM version is still not an internally-focusing model, and thus the end of the lens will still rotate (and can affect filters, etc.)
I'd recommend the IS version if you have the extra funds, but if you are on a budget and just can't wait, I saw less of a difference than others have between the two. Of course, the real step up would be to a comparable piece of glass in the L series, but with it comes a real step up in price, too.