...on the classes of user you have in your estate.
VDI/App-V/XenApp can bring a truely portable desktop and a real improvement in service to some classes of user. For others a mix of fat/web clients is always going to be more useful.
If you're looking to establish the tangible benefits of any scale of virtualisation project, you've really got to have comprehensively mapped out the user/application/hardware estate and it's dependencies.
For many organisations this piece of discovery alone could require as much capital as a desktop refresh with a bit of consolidation and rationalisation tacked alongside as a low cost 'added value' component of the project.