Depends on the network
Many people think about the newest, shiniest parts of any infrastructure without giving some serious consideration to that old chestnut, that the pace of any group is the pace of the slowest member of that group. So if you have a dozen VM desktops running off a shiny new IBM BladeServer, with MS Windows 2008 R2 and a brand new app server, it's still going to underperform if you're using 100MBps Cat5e crossover cable. It's the pinch in the hourglass, and the reason why the 'cloud' hasn't exactly been popular amongst end-users.
Desktop virtualisation will only become viable, reliable and stable if all areas of the infrastructure are upgraded to support a greater amount of network activity comessurate with the increased demand from the client terminals.