I'm very pleasantly surprised by this. I've always been a bit cynical about Basecamp's success, but I think this new version is going to be a genuine game-changer.
I really love the sheets idea and I'll love seeing how it works out. I'm already thinking about how I could apply the idea to e-commerce sites, hopefully the interaction isn't patented.
Thinking about it now, I am wondering about a potential limitation. Sheets make sense when you have a fairly consistent starting point, ie the project overview page. But what if you're given a link to say an individual ticket? Do the sheets build up from that ticket as a starting point, rather than the project page, or do they reflect the hierarchy of the site? If it's the later, does that not mean that loading a deep page in the site is burdened by having to also load its parent pages? I assume it would be handled asynchronously, but it still seems like a lot of overhead.