Disks that are connected via IDE (PATA) and SATA (including eSATA), support Self-Monitoring Analysis and Reporting Technology (S.M.A.R.T.) and report that S.M.A.R.T. data via the standard Microsoft storage driver (most modern PATA/SATA drives do), are fully supported.
After installing the add-in I'm now aware of one drive in my WHS that's 'degraded' and needs to be kept an eye on.
Seems a useful tool, and it's free.
For more about it visit the Dojo North Software site or Home Server Land