I've only used libvirt and virt-manager for about an hour an so far I have to so say it is totally awesome!
Well finally I did run into something a little wacky... it appears that it is very root-centric, meaning that I'm only able to use Usermode networking unless I launch virt-manager as root.
Might I avoid this by adding myself to the netdev / libvirtd groups? We'll see shortly.
Doesn't appear to be happening. I'm guessing this is better setup with a machine that does not use NetworkManager.
I hope to do a comparison of libvirt and Eucalyptus very soon.
UPDATE: I won't be able to review Eucalyptus as we ended up choosing VMWare instead. I would have preferred an open source solution, but the decision was not mine to make.