I know this is about a month old but I'll chime in and say I use the 100mm IS which I got used on Adorama. They have a pretty good rating system for used equipment and if you keep an eye out you can get a good deal on quality equipment.
this is full frame, no crop (except for a sliver at the bottom to remove the tank edge) from this morning. lil stellaria-6 by Peter Young, on Flickr