Ok, LuFS is pretty fantastic. Unlike all other other userfs implementations that I've come across it actually works, and it's fast. Using its localfs module (which just tests the LuFS interface by mirroring the existing filesystem) the speed difference is epsilon. Certainly for networked filesystems the LuFS latency is swamped by the network.
It's FTP and SFTP modules really work quite well, I certainly expect to be using the SFTP one when I'm back at Imperial.. It also supports autofs mounting so that you can just cd into (say) /mnt/sftp/agl@somehost and it will sftp (or ftp) mount it for you on the fly.
And the localfs module could be a wonderful way to chroot some difficult programs by mapping on a configured set of directories read-only. Though it would need some of the grsecurity kernel patches for it to actually be secure.