Until we installed Lion on one of the Macs. For some reason, mysterious network errors starting showing up and the sync would fail. Apple has replaced the SMB code in Lion and perhaps that's the reason. I sent a message to 2BrightSparks tech support and got a prompt reply which said (I'm paraphrasing), "Actually, we don't support Windows-Mac syncing." Oops.
For an unsupported feature, it had worked remarkably well and we weren't keen to throw the product overboard and look for an alternative. I remembered that SyncBack also supported FTP as a protocol for the sync. Sounds good, went back to Lion to turn on FTP and ... it's not there. Turns out that Apple has removed it or obscured it or something in Lion, because it is not very secure. (It isn't.)
But SFTP, a more secure but still standard, version of FTP is supported in Lion. Just go into System Preferences and check the box for Remote Login. Then go back to SyncBackSE and check the box for SFTP and ... it's disabled. Turns out you need SyncBackPro. OK, got that, checked the box, entered the SSH credentials and it all works.