I tried this a while ago with libbirt-kvm and the most awesome thing was that I could live migrate VMs from AMD to Intel and vice versa. None of the other solutions, like vmware or xen do that, afaik.

On the not so good side is that disk cache needs to be disabled for live migration to work at all. This reduces the iops of the vm by 50-60%.

So you either get high performance or high uptime...