75% of users report using VMWare today, and nearly 2/3rds report having tested an alternative hypervisor with Microsoft Hyper-v and Citrix Xen most often mentioned. Of those who have tested an alternative, 27% plan to use the it, while an additional 20% report that they may use it. Only 2% of VMware customers plan to switch to an alternative additional 9% considering it.
Microsoft and Red Hat has just announced that they have completed the certifications of the cross OS hypervisor agreements that the companies originally announced back in February of 2009. This means that Microsoft now certifies Red Hat Linux guests on Hyper-V and Red Hat certifies certain Windows guests on KVM. Red Hat has an excellent article on its web site that details which version of which products work with which and which provides an excellent FAQ.