Alessandro did a great job capturing the current "Ours is better! No, ours is better" fight between VMware and Citrix. (If you haven't read about it yet, the short version is that these two companies are fighting about whose hypervisor can support more VDI users.) They're going back-and-forth with test after test and insult after insult. At some point you have to step back and chuckle. Seriously? Arguing about which hypervisor can support more VDI users? This is like arguing whether a 1999 Ford Contour has better cupholders than a 2001 Chevy Prizm.
The only people using VDI today are people who have a tactical business need that does not allow them to use a Terminal Server-based solution.
Of course one could make the argument that "Sure, we don't choose VDI because we want good performance. But if we *must* go with VDI, then why not get the best performance possible?"
Good point. Except that the performance differences between the two products are more-or-less marginal (speaking from the viewpoint of an objective third-party who knows that anyone can make any results look great in their favor).
The bigger point is that VDI is such a huge undertaking for companies that the factors that weigh into the decisions about which platform will win out have to do with managebility, end user experience, application compatibility, etc. I don't think anyone is actually making buying decisions based on which of the two VDI platforms can support more users.
And by the time VDI gets really popular next year, we'll have new hypervisors all-around, so we'll just have to start this new game from scratch.