I used this (included in Office 2011 for Mac) for several months, and was very happy with it. I established a VPN session, then could connect to any of the Windows 7 or Server 2008 servers in our office or our site, quickly and with no problems. I'm using a 27" iMac, and even using 2520x1440 for my workstation window, I had no speed issues.
It did crash once or twice during that time while I was using it - I shrugged, reopened the session, and continued. More irritating were the times it crashed when I closed the session by clicking the close button on the window. The window hung around, black, until I force quit RDP.
The other irritation was the opaque way it handled stored connections, and routinely overwrote them.
The reason I had to stop using it, however, is that our IT department now rejects attempts to use VPN from non-corporate machines, so I have to RDP through a gateway. There appears to be no way to configure this client to use a gateway, so I've had to fall back on the version on my Windows 7 VM, which has full functionality.