I know one fellow who built a large private cloud for his organisation (about two racks of topline gear) so that they could simulate an architecture of several dozen servers and thousands of complex, concurrent transations.
What's it built on? VMWare. He was not totally amused when I sent him a link to Vagrant.
(I understand VMWare have similar fleet-orchestration tools, but not development-oriented as vagrant is -- awesome for production though).
Putting aside my friend, I can also see performance gains from in-kernel virtualisation schemes such as KVM^w lxc or Solaris Zones.
Bravo to the Vagrant team. This tool has become one of the centrepieces of how I personally do development.