I bought one of these last week and I can confirm that the unit DOES power down if not used for a period of time, even when it is still connected via Bluetooth. I'm not sure how long it needs to be idle but I fell asleep listening to something yesterday and when I woke up, the One had switched itself off.
I can also confirm that it is definitely the best sounding portable speaker you are likely to find. I agonised over the price but so far I am really glad I bought it. There area few little niggly things with it - all of these little speakers have compromises - but I think they are easy to live with when you hear how great it can sound when you hit the sweet spot (don't over-drive it).