I get the concept, that it lets you get rid of carrying around extra pieces of paper (coupons or paper tickets), and can be a substitute for another piece of plastic (credit card/gift card) too.
But like other people said, it's still half-baked. If you have to download another app (like Target does) just to put passes into Passbook, that's pointless. Why not just let the app manage its own offers/purchases and save the extra step?
And really, I'm not sure I even *want* all this stuff trapped inside my phone or device? I mean, say I have a bunch of credit for items in Passbook but then I drop and break my phone? Now I've also cancelled out my ability to access those funds/credits until I get the phone replaced and get my data synced back onto it? Same problem if my device is stolen or decides to malfunction, or the battery runs down and I don't have access to recharge it. People might say, "Who cares?" about a Starbucks coffee, but it sure would be an issue on a non-refundable airline ticket I was about to use!