Quest thinks HyperCache rocks.BrianMadden.com thinks it is great too but there might be a but - look at that but …
A few weeks ago, Quest released vWorkspace 7.5, which is a pretty massive update for only being a half-release. In the release, they rewrote the Web Interface while adding support for Citrix farms (since they see mixed environments and those users wanted one interface to apps), integrated the Desktop Optimizer into the management console, and developed a solution that allows people to use Microsoft Communicator or Lync across the WAN, among other things. Perhaps the biggest thing, though, is their Catalyst components.
Catalyst is the term Quest is using to reference two tools that they've developed for optimizing Hyper-V for virtual desktops (or virtual RDSH servers). The tools, dubbed HyperCache and HyperDeploy, add functionality to Hyper-V that allows it to cache frequently used storage blocks in memory, while HyperDeploy is sort of like a disk streaming solution (but not really) in that it allows you to boot VMs before the virtual disk is 100% copied. You can read more about it in Michel Roth's explanation.
The combination of these tools results in a surprising performance increase when compared to native Hyper-V and to other platforms. Quest posted the results of their testing in the form of a PDF, and while it's vendor-created, it at the very least shows that Catalyst has a positive impact on things. In the document, they outline their test procedures for each platform, so you can evaluate the validity of the results yourself. They did use Login VSI, which is the industry standard for VDI benchmarking and testing.
HyperCache works by dedicating part of the host server's RAM to serve as a storage cache. In this cache is stored the most used blocks from the golden (master shared) image. This means that requests for those blocks are served from memory, which drastically reduces boot storms (which are mostly read-heavy) and increases boot time. The size of HyperCache is only 800MB by default, but that can be changed. According to Quest, the entire Windows 7 boot process only amounts to about 350MB of data in their tests, so the cache is more than enough.