I recently uploaded my clients from 10.7.7 to 10.7.8 and since then have been experiencing repeated issues. The first one being the client seems to hang/freeze, becoming completely unresponsive, showing the spinning blue ball and the only recourse is to force quit the app. This can happen at any time and does not even require you to have any active RDP connections. Even just pulling the client list down from the menu bar icon appears to be enough to trigger the behavior. The second one occurs when the setting to pin the menu bar option so the client list does not display in a separate window. In past versions, it is possible to click the Microsoft Remote Desktop icon in the Dock and the client list unfurls from the menu bar icon. This action no longer works and even when you click on the menu bar icon the first time it appears to recognize the click and for a split second you can see the list unfurl but it immediately disappears until you click on the icon a second time. Rolling back to 10.7.7 resulted in both problems to go away immediately.
The more I've tested in version 10.7.8, the more it seems the above issues only occur when the pin to menu bar option is set & you click on the Dock icon to unfurl it a few times before realizing it will not unfurl that way anymore. Then when you do click on the menu bar icon and get it to unfurl you can try single clicking on a few of the client entries before the client will hang and start showing the spinning blue ball and requiring a force quit.
So for now, the workarounds appear to be:
1) Roll back the application to version 10.7.7
2) "Unpin from menu bar" in the application's settings
3) If you have set "Pin to menu bar" in the application's settings, do not click on the dock icon to try and get the client list to unfurl. The application seems to run smoothly for me in this setup but once I click the dock icon 3-4 times, things go sideways shortly thereafter.