Microsoft Remote Desktop v8 has been out for almost a year, available through the Mac App store, and has been updated several times. The current v8.0.9 fixes high CPU usage introduced by the v8.0.8 update.
The initial release of Microsoft Remote Desktop v8, and several subsequent updates, were compatible with OS X 10.6.8 or later. I’m not sure when, but at some point an update dropped Snow Leopard support, and currently Microsoft Remote Desktop v8.0.9 requires OS X 10.7 or later. If you are successfully using Microsoft Remote Desktop v8.x on a Mac running OS X 10.6.8, do not apply any updates.