hey doc - I am deep into this as well at Webb (my company). I am going to be pitching it to management for a next year project so I have been reading alot about it.
The 2 that I like are Xen and VMWare. MS's product is terrible in my humble opinion mainly becase it requires you run it on Server 2008. To me, this means reinstalling all my apps and having to start over. not an option on a network of my size or complexity. Also, the footprint for MS's virtual is 2.8GB while the VMware solution is only 32MB and runs on Linux - so its 100% stable.
That being said, to me VMWare will win my thinking for one factor alone...
it will migrate a hardware server into a virtual environment as a part of the install process. That right there is the single best feature I have ever come across. I dont have to reinstall anything. I migrate, bring it online and I'm done (in a perfect world of course).
I will post more info tomorrow also.
any questions so far buddy?