I brought the newer IS version. Also got a Manfrotto 454 sliding plate to go with it. I've found the plate is a very handy tool to have since the DOF is so tiny. I attached a QR plate to the 454 for quick change over, but i need to manufacture a space plate for the QR plate so i can position it better.
I can see why macro photography becomes an obsession for some people. I'm not sure my mental state will cope with moving onto living creatures.

I tried image stacking in CS5 for a few shots. It was hit and miss for me. Sometimes the blend was spot on and other times it didn't work at all. It might be possible to manually edit it all, but working with 10 or more layers with their own layer mask piled on top of each other is far more than i'm capable of.
