A couple of things to caution about here also.
Don’t confuse the older 75-300 with the current 70-300 lens. The new one stands heads above the older one, and can come close the the quality of some L glass, you can check out the review of it here: http://www.photo.net/equipment/canon/70-300is/review.html
The IS on it is great, as I can hand-hold 1/30th of a second at the full 300mm with it.
However, as said above, you will get better quality from the similarly priced 70-200L f/4 (When wide open, at f8, the two look near identical). It doesn’t have IS, and doesn’t have the extra 100mm of reach, but it will focus faster, and is better built.
Since I already had the 70-200L f/2.8, I got the 70-300 so that I always had a long zoom with me, since the L is way too heavy to cart around daily.
The DO, I tried, and it got worse results then the non-DO (for me, YMMV) also its’ bokkeh(background blurring) wasn’t as good. 28-135 is a decent lens. better then the kit lens, but amazing it isn’t.