I've been using ESXi server for a while now. It's a pretty strong machine (i7-2600 + 16GB RAM), and I use it to run concurrently 5 VMs (Mostly Windows Server 2008 R2).
Most of the times, the VMs run just fine. However, my main issue is - when one of the VMs has extensive datastore usage (usually from applications left open, or extensive memory swap if virtual memory is running out), it makes all of the other machines extremely slow as well.
I tried using resource pools - but it seems like there is only CPU/Memory restrictions and not disk usage of some sort. (I'm using the free version of ESXi - don't know if premium versions contain this feature).