Hi,
We've been using Proxmox for the last 5-6 months.
Our setup is of 4 nodes in HA cluster. With a mix of KVM and OpenVZ, local and network storage.
We found it to be a very stable and configurable solution, and very easy to manage (via GUI or command line).
There are some issues (minor) with the packages included in the iso, that there was no way around, but as far as performance, reliability and free support, we're very happy overall.
em.