I had own this 70-210mm f3.5-4.5 before I upgradeed to 70-200mm f2.8 L 2 years ago. This 70-210 is a good compact lens for almost any kind of photography. User reviews actually varies from people to people. For me I rated a 8/10, for its sharpness at f4.5, f5.6 & f8. It is actually a L lens in disguise.
The only draw back is the zoom-creep - when lens is pointed downwards, zoom barrel will 'pop-out'. Fear not, as a past user, I did some cheap mod to the barrel. Add a thin layer of masking tape to the zoom barrel. The extra layer of tape will keep the zoom barrel in place. Change the tape occasionally.
http://www.phototestcenter.com/html/..._f3_5-4_5.html
http://forums.dpreview.com/forums/re...ssage=10733765
http://www.greenspun.com/bboard/q-an...?msg_id=008KUm
Compared to 70-300 IS, of course got IS is better, more cosy but not necessary better. I am sure you will be happy with 70-210 for most of the occasions.
This lens work well for 1.6x cropped bodies but doesnt work well on 5D/1D series. Thus when I got my 5D, I sold it to upgrade to 70-200mm f2.8 L. If it work well for my 5D, i would have keep it. But it had serve me very well.
Cheers