Well, so far I've got as far as downloading the files, installing the application on a couple of test machines (You don't think I'm brave/crazy enough to roll this out on production machines, do you?), set it up and thrown a few test viruses at the app. So far, so good, but I've still got a lot of questions:
What's stability like?
How quickly will the app respond to new threats?
How much of a performance hit is the app on systems, especially when running scans?
How long until hackers start busting holes in the app?
What will other security vendors make of it?
These and more questions will be answered at some point in the future ...
So far though, so good. Microsoft Security Essentials seems pretty primitive (no email integration, for example), and I'm surprised it doesn't integrate better with Windows Firewall. That said, basic antimalware software is much better than none at all, so it's hard to complain about it.
By the way, if you haven't downloaded the installation files for Microsoft Security Essentials beta, you've missed the boat now because Microsoft closed the door on downloads.