I played around with Red Hat’s virtualization on RHEL6 last weekend. Color me impressed. I had looked at libvirt a couple of years ago when it was in heavy development in Fedora, but I was amazed at how far they’ve come in that time. It has become very polished.
I was able to easily fire up a RHEL6.1 beta guest and clone it (though it can only be cloned while paused or shut down, unlike VMware). Performance was very snappy, and an idle guest used very little resources. My project this weekend will be to set up a second host and test out migration, play with virsh (the command line equivalent to virt-manager), and see how RHN registering works on guests.