We have a NAS setup with NFS configured with ESX v4 and running VMs. If you are going down this route, here are a few tips:
Have redundant NICs for NFS traffic and a pretty good backbone back into your NAS (we use dedicated gigE). You can also configure a vSphere host with a dedicated NIC for storage traffic.
vSphere gets finicky with misbehaving mounts. If you lose connectivity to a NFS share for whatever reason, the mounts may go offline and the only way to bring them back online is by a host reboot, recreating the mount from scratch, or running some command line Vmware scripts on the console - could not get do it from the GUI.
This post has some interesting discussions on parameter settings for ESX 3.5 and NFS mounts - you may want to check if some of these are applicable.
Other than that, works pretty well for hosting VMs, snapshots and vmotion.
Hope that helps,