Yes, it's installed on an external drive. I'm crazy, not stupid.
First impressions: it's very pretty. It's fresh. The Yosemite-era UI is now six years old and was looking a little long in the tooth. The new design is probably as radical as the transition was from Mavericks to Yosemite and of course people are already bitching about it; about the "iOSification of macOS." Deal with it. Life marches on and no one's going to force you to upgrade if you don't want to. As to the experience of using it right now, it's rough—very rough. Lots of things are broken/don't work. (Which I've been dutifully reporting back to the mothership as I run across them.) I now understand why it took so long for Apple to release this to the public beta testers. I can't imagine what the earlier developer versions were like.
I DIDN'T install it on an external drive because every time I wait for the Public Beta and always find that any problems I have are minor. Big Sur is a radical exception to that rule. I am having horrible wake-from-sleep issues on my new MacBook Pro 16. The battery consumption is insanely bad. And basic features are busted… like just when you force links to open in a new tab and they are ALWAYS active even though you have it unchecked in prefs. How does something like that get unnoticed by past beta testers? And while I think the overall look is nice and fresh and something I can adapt to, the icons for Apple's apps are butt-ugly. And don't get me started on the menu bar. Holy crap what a mess. Menu bar apps are spaced so far apart that I had to uninstall some that I was using. You can't even tell which are active because of the reverse-out which is awful. I sure hope that things improve… and improve a LOT… by final release.
I installed it last night and have been playing around with it here and there. I haven't found a lot of broken things yet. Aside from the visual refresh I'm not finding a huge number of changes from Catalina. I am surprised to see how fast it's running, even in this beta form. I have it on my tricked out 2015 MacBook Pro (before the keyboard debacle started) and it seems happier than Catalina on the same machine.