Every time I see someone pull out an S4 I'm stunned. Sorry, but it's true. The way they tapered the sides with the silver it looks way thinner and more elegant than the iPhone brick. Also I think the screen proportions are different. I like them better. The gigantic size I could take or leave, but I'm not surprised it's a hit.
I haven't used passbook once. The whole experience just feels odd to me. Having a wallet I don't understand that might automatically open unbeknownst to me when I arrive places sounds... Like something I'd rather not deal with.