We've tried it at work and found it to be okay...
Ran the Microsoft product on my machine, which is clean, and there were no false positives. Spybot and Adaware showed the same result.
A colleague ran it and had two, on legitimate software, however, it appeared that it simply didn't recognize the software and flagged it.
Seems to have some nice features, but the jury is out because it is a beta version and they probably will start charging for it.
It will not work on 98 or ME and has two known issues. One is a conflict with SMS, which only affects networks that use SMS and won't affect home users.
The second is with Windows Media Center Extender...info here... http://support.microsoft.com/?scid=kb;en-us;892374
It may be worthwhile as an adjunct to Spybot and Adaware, but not alone.
Check out this SecurityFocus column and responses for more information and comment...
http://www.securityfocus.com/columnists/289