I've been using Windows Live Sync for about the last month. I have to say, I'm impressed. I customarily use three to four computers throughout the day: my tablet, my desktop, my work MacBook, and my Ubuntu server. While I haven't had that many computers to keep synced the whole time, I've been looking for a good sync solution for the better part of the last four years.
I used SyncToy for awhile, but it had to be run manually and got very complicated when syncing more than two computers. It did have the benefit of working with Linux in that it could sync to network shares (it didn't actually run on a Linux system, but could be used via Samba).
I would have liked to use Microsoft Groove's folder watcher, which works in a completely peer-to-peer(other than using Microsoft for authentication), automated fashion, but Microsoft has deigned not to make the folder watcher component functional on 64-bit systems, of which I have two.
I held off trying FolderShare/Live Sync for a while because it limited it each partnership to 20000 files (yea, i know that's a lot, but I had a large number of Subversion controlled projects; version control produces a lot of excess files). After compressing a number of the projects that I haven't touched in awhile, I got reduced my file count and tried Live Sync. Surprisingly, it works. I've got it set to sync my desktop, my tablet, my MacBook, and another laptop that I use occasionly. Granted, it doesn't work on *nix based operating systems, but I don't really work with documents on my server (and I have a different tool doing backups on the server anyway). So far, the only complaint I have is that when the Microsoft server goes down, there is no method of authentication and Live Sync stops working.
I found another project, lsyncd, which seems to provide the same functionality as Live Sync, but does not require a central authetication database. Unfortunately, lsyncd must be compiled from source and doesn't have any easy Windows support (i.e. it can be compiled on a Windows machine, but that means getting all the dependences and probably setting up MinGW, make, binutils, etc). It looks like there may be a way to run lsyncd on Linux systems and have it interface with rsyncd on Windows systems. I'm not ready to switch to this project yet, but I will be keeping an eye on it.