If you're migrating from one store to another with any more than 10 products or so, you've got to use this. I migrated from Xcart to Magento with 100s of customers & products. I probably could have done this myself by mapping out tables, fields, etc., but that could have taken hours. I tried Cart2cart, and it took maybe 20 minutes total. I totally didn't expect it to work... with everything that could go wrong, surely something would. But it actually worked amazingly. There were a few fields that didn't move over, so I wrote Cart2Cart to see if they could include those... not only did they revise their migration process to include those, but they then redid the migration for me. Very responsive support team there, and in private correspondence, I can tell they are serious about making improvements wherever possible.
Joe Fletcher