I've checked out MirrorFolder
a bit, and it seems very very very
As far as I've been able to tell, the best the other applications can offer is detecting modified files via filesystem events - this has two problems. One is that Windows' fsevents only tell you that "there's a change somewhere in the tree you're monitoring", not WHAT has changed - which means you have to scan the entire tree upon notification (unless you resort to dirty tricks like reading the NTFS journal). The other problem is that you have to synchronize the entire file, even if only part of it has changed (probably not too big a problem for my uses, but still...)
MirrorFolder is cool, though. It can install a filter driver that monitors changes, so only the changes are synced - ie., changing 10 bytes in a 10gig file only pushes those 10 bytes to your sync destination. And it even works with network shares (couldn't find a way to set user credentials, but mapping at network drive worked like a charm). The driver is 73kb and the server is 118kb, so it's not exactly bloatware either.
I will have to do some further testing of this, but MirrorFolder seems like a natural winner for my purposes, if it doesn't impact performance too much, and (of course!) is stable. But I expect the filter driver approach to have pretty decent performance. The only nag I have about MF is that they call the filter drive method for "RAID-1", which is wrong and misleading and doesn't give the technique enough credit
Still, if anybody has other suggestions, keep 'em coming!