Can't vouch for LXC or VServer, but OpenVZ is well documented and I was up and running with it within days. I still have a dev server that's been happily running for about 5 years. Users get root access to a 'container' which has some limits imposed upon it. They can do as they please. I don't have to worry about them messing up the host (, they are trusted users though.) The file system is easy to back up, as it's reachable from the host. The server itself doesn't support hardware Virtualisation so it's a nice fit.
