It's bit useless in the UK. Apart from hotels/booking/starbucks and the airlines, there aren't many shops using it yet, which is a pity. My wallets always, no matter how expensive, always end up lasting less time than they should because of the amount of clubcards I carry with me. Most of them are from fairly popular franchises in the UK. I don't know why they don't take more advantage of this technology.
But then again, not even the Eurostar app has been updated to work with Passbook, so... I think Apple needs to push this to retailers and convice them "how it the most amazing thing ever".
And c'mon! It may not be the most amazing thing ever, but it's surely incredibly convenient.