I upgraded my lab server this morning from MR4 MP2 to RU5 without issue.
I configured an install package with Antivirus/Antispyware,NTP, Truscan, and application and device control for the 32 bit client to push to a group containing 4 Windows 7 RTM boxes that were running the MR4 MP2 client with Antivirus and Antispyware ONLY (since that's all that would work under Windows 7). I configured the package to NOT maintain existing client features and settings.The pushes went flawlessly (and quickly), however, after the install, all of the boxes showed Network Threat Protection as 'waiting for definitions'. This never went away, even after reboots, forcing content updates, waiting for over an hour, etc. Other than that, SEP appeared to be functioning properly, including application control. Antivirus and PTP definfinitions did update without issue.
Installing the new package via the deployment wizard OVER the existing RU5 installation did not solve the problem. Uninstalling SEP and LU, rebooting, and then installing the package via the deployment wizard DID solve the problem, and NTP definitions update successfully.
I've seen another small bug in the SEPM. When I run a command, such as restart, on select clients in a group, the Monitors page is showing the wrong number of clients in the results. For example, I chose to restart two clients, and the monitor logs show that I selected 5 clients. When one client finished, it showed 60% completed. When the other client finished, the remaining 40% was reported to be finished. I tried this with a couple of different commands in two different groups with the same results. All the clients are Windows 7.
So far that's it. Now it's time to see if system lockdown has been fixed. My Symantec tech support person reported that it had been fixed in RU5, but I haven't seen anything official yet.