I have many customers running it and loving it. It performs very well. 

Performance: If you have a very large Exchange 2007 deployment, you'll need to be aware that you'll need to scale out with multiple Exchange VMs rather than expecting identical performance from a VM.  Here's a good pointer to a test from March of last year where they got 16,000 mailboxes on a single server by scaling out on VMware: http://blogs.vmware.com/vmtn/2008/03/16000-exchange.html

Licensing: check with the horse's mouth: http://www.microsoft.com/licensing/resources/volbrief.mspx

Storage: Whether it's SAN or DAS the same requirements exist that existed previously.  Make sure you have the necessary amount of disk spindles to drive the IOPS requirements of your installation. 

Enjoy! 

Sean Clark - http://vmworldunderground.com - http://twitter.com/vseanclark

If you intend to virtualize Exchange 2007 then you should probably follow Microsoft's recommendations for doing so.  They can be found here:

http://technet.microsoft.com/en-us/library/cc794548.aspx

Note that ESX is part of the Server Virtualization Validation Program and therefore Exchange is supported on it.  Just replace the words Hyper-V with VMWare ESX, pass-though disks with RDM, and so on.

Also note that you should not use snapshots nor HA with a virtualized Exchange 2007 deployment.