I bought my 75-300 IS USM from a local dealer as-is. It was an unboxed trade-in. At the time the retail on a new copy was $960: I was there to buy one. By quirk of fate a guy had just had this lens brought over (to the location I shop at) to look at, and he passed on it! I snapped it up at $550. I felt like I'd stolen it.
The lens is sharp, the focus is accurate, and the color excellent. It did exhibit chromatic aberrations on things like water droplets in the sun, but those were easily fixed in post-processing. It has nice bokeh, especially when doing macro. Having used the non-is version of the 75-300, I enjoy its nicer feel and overall build quality.
The only complaint I have is the focus is a bit slow, and in low contrast or low light levels it will either hunt, or refuse to focus at all. On the other hand, in decent light, when coupled with an extension tube, it focuses on small subjects accurately enough that I seldom have to manually focus it.
For birding it was amazingly sharp, but it doesn't produce decent panning results unless you remember to switch off the IS (I never remember to).
I'd buy it all over again. Technically, I did. I gave my copy to my girlfriend and grabbed the generation 2 version (the 70-300 IS USM). I bought that one new for $699 CAD. It's outstanding! Faster, more accurate focusing and better color. I figure my girlfriend will have the 75-300 for a long, long time.