quick update: my OnePlus One broke the same day I wrote the last post. I have gotten an OnePlus X since.
It is basically the same as the One, but in a smaller, more elegant package. Camera, slightly worse, AMOLED screen much better, battery slightly less capacity and a few hours less life.
I recommend it at sub-300 € (although I got a Ceramic edition, very nice, but not really “worth” the 90 € price difference. (company phone, otherwise I would not have got it)