While I can see the point of Basecamp ("The simplest thing that could possibly work" for project management), Backpack seems utterly pointless. It's an "almost wiki" where each wikipage consists of multiple data items, combined with a TODO list. Pages have access controls and there's a simple email=>wiki update feature where you can do an edit to a wiki page by sending an email.
Augmented wikis are done better elsewhere.
Parts of the application simply shouldn't have been released, since they are clearly not done yet. The "email this page" feature is horribly implemented. First of all - to email a page you have to use a special "email key" as the address. Having a hard to remember email address to keep track of outside your personal organiser application kind of defeats the purpose of an organiser, no? There are simpler and better ways to handle the security issues of email, e.g. the way JotSpot does the it. And when you do send an email to a page, what you get is an embedded subject line, which links to a page with this inviting look. Incidentally, I did not set up this page to be public. Access control just isn't working yet for embedded email.
There's no easy to find search for your data. I thought GMail had made that mandatory for this kind of application.

The most impressive thing is the hype/content ratio on this project - almost enterprise grade. The hypefest here really tested my gag reflex.
But the worst thing is actually this: I don't think the 37 signallers realize they just created a "me too" product of the worst kind. There is nothing new here that isn't in several open source packages and/or one of the other social software products. No convincing extra simplicity, no fresh new UI ideas.