I'm beta testing IE8 right now and not having any problems. I wouldn't touch IE8 beta 1 with a 10 foot pole. IE8 can be uninstalled from Vista, or didn't you know that.
Glad to hear you're not have trouble with IE8. And for many people it may work just fine. What if something goes wrong. The system restore fails, the backup fails, the uninstaller fails. If software is in beta, I am presuming it's the whole package, including the uninstaller. What if there's a bug there, then what? I just offered my opinion on the issue. Caveat Emptor if you want to use it. I've seen someone use a pre-release IE7, toasted his system and the uninstaller failed, no rollbacks, no backups. He had to do a complete reinstall of the OS to get clear of the failed IE7 when it was pre-release. Now IE7 is widely used without any issues. And someday IE8 will be the standard. The question is: Do you want to put your system at risk for Microsoft's "testing" software? Do you have that much faith in their uninstaller, that you believe it will fix everything that can go wrong? Personally, I've done the dance with betas of various flavors, and have paid the price (in failed systems). At this point, I'm not willing to put my system on the line for a software test. I'll let someone else deal with the flaws, and when the public roll out is available, then I'll try it. Like I said before, my .02