Has anyone tried it?

I just read about it, and it sure sounded awesome. I'm not sure why has it not gotten much media (tech media ) coverage. Ext4 was not nearly as exciting, and it got a lot more coverage.

NILFS just came out with the 2.6.30 kernel, still marked experimental.

Essentially, it's a log-based file system with CONTINUOUS snapshots. So you can, for example, mount your filesystem at the state it was in 2 days ago. Read-only, of course . This way you can undo any unintentional changes. Undelete, unmodify, etc.

"Check points" are taken automatically and very frequently (every few minutes). They are garbage-collected by a user-space daemon after a while. The user can convert check points to "snap shots" to make them permanent.

Snapshots can be mounted at the same time as the "current" writeable fs. That can be very useful for making backups.

This is even better than Solaris's ZFS (I was seriously contemplating trying Solaris just for ZFS). ZFS also allows snapshots, but AFAIK it's not continuous (modifications between 2 snapshots are lost), and users need to make snapshots manually (exactly how many people make snapshots before running "rm -rf /"?).

Since I have 2 weeks on my hand... I think I'll play with it on my laptop.

I think Linux is the only OS so far that has anything like this. BSD has a read-only driver for NILFS. Solaris has ZFS. Windows is still on NTFS from... 15 years ago?