I installed w2k8/64 and exchange 2k7 on a vmware esx (kept waiting for hyperV, but even when it came out M$ would not say it supported exchange). There have been many issues along the way - I did not think that doing the upgrades six months after the release would be so 'bleeding edge', but it seems to have been. Even late in 2008 some third party apps were not supported on server 2008. I want to move forward with other server upgrades, but others recommend keeping stable apps running on 2003.
BTW - exchange 2003 to 2007 did NOT go smoothly - I am still suffering from issues of decommisioning the 2003 (public folders, GAL, OAB).
We don't/won't use Vista so there is no advantage seen there. I have yet to setup the TS (which I understand is probably the biggest improvement).