i'll throw a little opinion out here, as i shoot canon, i feel somewhat qualified
how soon do you need/want this lens, i ask because if you can wait, there are several very good otions
1. check out
www.keh.com , several of us forum members have been satisfied customers over the years purchasing used lenses from them
2. the lens you might want is the 70-300 f4.5-5.6 IS, this is a great lens for the price, i just checked keh, they have several for around $400, in excellent shape, and keh is notoriously conservative on their grading. if you are interested in this lens, go on over to my site, to the 2007 med cruise gallery, i took all the tele shots in europe with this lens mounted on a rebel ti
3. really save those dollars until you can get the new 70-300 f4.5-5.6 L IS, this is the replacement lens, just came on the market last fall, i went to b&h to look, handled it in the store on a 5d, took a few test shots, and went home with it that day, right on the spot. downside, it's $1600. but the sharpness, and the focus speed are just spot on. i still have the older non L lens, it's in good shape, i might be willing to do a deal.
4. stay away from the older 75-300 and the non is 70-300, those were really inferior product, the 75-300 had one of the shortest product lifespans ever for canon, i think within 2 years they put out the IS model
hope this helps, remember GOOD GLASS, TAKEN CARE OF, LASTS A LONG TIME AND HOLD ITS VALUE
