What lenses do you have already and why are you interested in IS? IS isn't designed for photographing moving objects, what are you intending to shoot with it?

The low end IS lens is the 28-135IS. A VERY good walk around lens.
Then there's the 70-200 2.8IS (the best of the best)
Then the 70-300IS. (eh, don't really like it)
then the 100-400IS (2nd only to the 70-200)

In my opinion, the mac-daddy of the 3 is the 70-200.

Canon lenses hold their value extremely well, a 2 or 3 year old excellent quality copy of any of those lenses will pretty much cost the same as new. You can check bhphotovideo.com, they have a used equipment section on their site, but nothing lasts long. You can also check out fredmiranda.com, he also has a buy/sell section.

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