I couldn't agree more, I got my 70-300 f/4-5.6 IS used off of Craig's List last Summer and I have to say I am thrilled with the results as well. I honestly don't think you can go wrong with it.
Don't get me wrong the 70-200's certainly have their place, I love mine as a portrait lens but honestly don't do many portraits or low light situations. I've considered selling it but haven't been able to part with it just yet.
I also agree 110% that IS is AWESOME!!! My first IS lens was my 24-105 and when I saw the results from that I was hooked. I knew it would make a huge difference on longer lenses as well. I think it is well worth the extra money, too bad Canon doesn't offer it on more lenses.