I suppose the fact that it's buggy as hell is a good sign. If the IE9 preview is crashing on things that IE8 could handle fine, that would indicate that they're making substantial under-the-hood changes to the way the IE rendering engine works. The alternative would be where it only crashes when using new functionality, which would mean that they were just adding new features on top of their legacy code. Call to mind the old NGLayout demo.
On the other hand, the fact that they'd push prerelease software out the door at this early a stage seems almost like a last ditch effort to say, "Look, our browser is still competitive!" even though at this rate it's still far from it. Prealphas and nightlies work fine on the community-driven model, where an open group of end-users are doing bug testing, but is kindof pointless when the development process is going on behind closed doors.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.3a3pre) Gecko/20100310 Minefield/3.7a3pre
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