PXE is very fast and very useful. We installed thousands of machines here with a PXE boot, using Debian's FAI, Kickstart and a modified BSD installer. It's definitely one of the easiest ways to install machines, especially when you need automated installations.
I've installed my last two personal boxes with PXE. It's especially useful with netbooks, since they don't have an optical drive, and I am quite reluctant to use a USB key to install my system when the network is quite fast.
I've also used PXE boot with LTSP (in Edubuntu), which allowed to set a park of think clients for a group of children in a very short time, just plugging old machines to the network and booting them so they would get their system from the PXE/DHCP/NFS server.
As suggested already, if you want to set a farm of servers, I would really recommend that you use a PXE installation system together with a class-based network installer (such as FAI or kickstart) and finish the installation of the machines by plugging them to a configuration manager such as Puppet or Cfengine.