Hi-ho,
My father has this lens, and I owned the 90-300 USM for a wee while when I coverted to EOS in 2004...
Firstly, I'll second what Leaf said about getting what you pay for....
With either the 75-300 or the 90-300 you need good light to get sharp results at any focal length. That's because you need to stop down at least one stop from wide open at any given focal length. (ie: F/8 at 300mm, F/5.6 at 75mm)
The focus on the 90-300 was glacial at 300mm even though it has USM. It's not ring USM though, so you're not getting the huge advantage that USM has the potential for... They are very quite focusing though, which is a plus. The 75-300 seems slightly faster to focus the times I've used it, but can't say for sure.
Colour and contrast is OK stopped down, but wide open at anything above 200mm or so the images need contrast & saturation adjustment to get them to 'Pop'. at 90mm my one was pretty good for contrast, although colours seemed to be a little cool.
If you're on a tight budget, get it. If you're not on a really tight budget, save your $ for a while and get the 70-200 F/4 USM L and you'll never regret it. (799.99 on henrys site). (Or get the 75-300 and start saving for the 70-200 F/2.8 :-) )
Just my 2c worth.