Yes, I installed Microsoft Security Essentials Beta. It's too early for an educated opinion. Testing an AV is not easy unless you have a private Zoo full of Virus and Spyware.
Still, I scanned my PC and it found two issues. One of them is a version of VNC that I use to remotely assist some clients. So even when there is some risk I would call this a false alarm in my particular case. The other one was a legit one.
Here are some screen shots of the program in full action.