Imagestation made it real easy to transition over to Shutterfly. Just follow the instructions - there's a link in the banner on the first page (and probably elsewhere). It only took a couple of days for my albums to move over. Some of the picture titles got garbled, but other than that, it seems fine to me.

I've also set up a Photobucket account for online sharing as I'm not sure how flexible Shutterfly is about cross-posting pics. Imagestation never seemed to care, even though technically it was against their rules.