I bought one a few months ago to support the project but never ended up using it. It's still sitting in a drawer.
Realistically, there's no hope I'll ever do anything with it myself. ;)
It's the most viable and promising so far I think. I have too many directories for it to be usable without some new features, though (namely highlight dirs with new mail and more filtering capabilities). It's practically unusable, but granted I may have an unusual setup. If you just have a handful of active folders it might be great.
Didn't notice any surprising syncing issues so far.