So far, I have taken four labs, and I must admit that I'm impressed. It works so well that you're not even thinking about what is going on under the hood. What you're actually experiencing is that as soon as you're choosing a lab, then almost instantly you have two ESX servers and a number of VMs provisioned for you. A VMware employee told me that part of the custom code is calculating the most popular labs, and then pre-deploying a number of these up front as to reduce waiting time.
Furthermore, the whole user experience is pretty cool. They've made a GUI for choosing the labs, and on flatscreens around in the room you can see statistics such as most popular labs and total number of VMs created and labs completed (see pictures below).