Seeing an email from Jason Fried in my inbox on Saturday was an unexpected surprise! Tiger on Friday and Backpack to try out pre-launch the next day?! WOW!
They requested that no screenshots be posted prior to tomorrow's release so you'll have to take my word for it: Perfecto. If it weren't for a trade show this week, I'd be transitioning my Next Actions and such to it in full force. That will have to wait. It's Wiki, it's blog like with page-by-page sharing control... and much more.
In sum, I'm learning to love Backpack and suggest you try it starting tomorrow.