Simple interface -- only the necessary options (like the merge option, btw) and uncluttered. Scheduling a very nice feature, too.

I've been using Ben's FTP server software for years on a production machine. His interface may not have a ton of eye candy, but the code is solid and functional.

The more fleshed out options like Synchronize Pro certainly have matured nicely, but the price for Sync Pro is beginning to disappoint prospective customers. The pricing on this tool, particularly for the consumer who has not moved up to Leopard or doesn't want to rely solely on Time Machine, using a sync tool like these is paramount in preventing unexpected data loss.