I got mine yesterday, while I am very picky about my phone's, I really don't want to return it.
First, I took it out of the box at home. I saw the legendary "gap" at the top especially, which could get worse in the future, let in dust or wven worse. Then I saw that both corners at the bottom of the phone aren't made well, since the aluminium plate that covers the botom speaker kind of goes over the polycarbonate sides. Put it in my pocket, those 2 sharp edges get stuck somewhere, I pull out my One and tada: I might pull of the aluminium plate of the bottom speaker.
Then again, I started using the phone for an hour and I fell in love (no joke). The thing that blew me away the moest were those speakers... OMG they are fantastic. I haven't been so impressed about a mobile phone in years. The screen was fantastic also (no dead pixels afaik). All in all I love it so much, I wish I could just keep it. But since everything is still pack nicely in the box (charger, earbuds, everything) I will just put the One back init's protective plastic (which is very easy btw, nice) and go back to the store.
Why not try to get a solid build? In the end, they promised zero gap.. Right? But really, I think it's a pretty "real" problem. Even my girlfriend (which also loves the device) said I should go back and doesn't understand why they released these device.. Since you're paying a lot for these products, you'd expect a better made product.
Note: most people wouldn't notice these problems. I'm sure 90%+ of the people I would have showed my One too, would not have seen it. So if you don't mind these kind of small things, don't be afraid!