Hyper-V has made great strides since it was first released alongside Windows 2008. If I were a SMB/small enterprise running mostly a Windows shop and looking into virtualization, I would definitely give it a serious look - it can do most everything I would need for a lot less than vSphere. However, vSphere is still the king as far features, specifically when it comes to availability and DR.
That said, the growth of KVM & Openstack and the community surrounding them excite me more than anything coming down the pipe for either Hyper-V or vSphere.
Disclosure: The company I work for is partners with several vendors - http://www.latisys.com/partners/strategic_partnerships.html