There's also Bittorrent's SyncApp to consider, though it's still in alpha:

http://labs.bittorrent.com/experiments/sync.html

Did it take them three years to launch? I know there is tech behind this but if it doesn't take off then they're fairly stuck.

more discussion here: https://news.ycombinator.com/item?id=5483600

Why would it be?

The service looks very similar to our Build-your-own- Dropbox service Tonido Cloud (http://www.tonido.com/cloud/).

We have evaluated Java for the client side like AeroFS. But decided against it and went with a native client.

AeroFS comes across as far more focused and specialized app with a bit of supporting service on the back. Tonido however looks like a universal something that likely includes a lot of core features that I don't need. I considered trying it several times and ended up walking away, because it was just too grandiose in its feature set and rather amorphous in presentation.