At least with a conventional helicopter there is the chance of setting it up for autorotation if the power fails - admittedly with some models one needs lightening fast reactions and a lot of luck. (I suppose with the miniscule size of the rotors on this aircraft it might be possible to have an emergency reserve parachute the way some microlights do.

What happens when the battery expires and you're airborne, or one or more of the rotors stops?

All very easy, and technically not much more difficult than building a model multi-rotor - just (a lot) more expensive. The hard bit is getting a useful duration out of the thing (batteries!).

"What happens when..." For goodness sake! You are clearly utterly unaware of recent developments in electric flight - I suggest you start your education here... http://www.pipistrel.si/plane/alpha-electro/overview
The video would be much more interesting *without* the irritating music but a very intriguing and apparently well-advanced concept with lots of interesting potential applications. I hope we hear a lot more from your company in the near future. Congratulations. MW

Employ a micro-turbine generator and enjoy long flight times using almost any fuel. Employ a tilting mechanism (little more than a pivot hinge) to a couple of the motors and enjoy pretty high flight speeds / efficiencies. Employ any number of form factors (think a little outside the "make it look like a helicopter" box, guys) to and enjoy far greater rigidity, versatility and utility. This is a wonderful first step, but let's take this thing and run with it!

@Nik It has 18 propellers, this allows it to continue flying even with multiple propellers / motors stopped.

@Mel Tisdale If a single main rotor helicopter damage or bend a blade ever so lightly, it will crash. Multi rotors like the Volocopter can damage multiple propellers and still keep flying.

I think that is really cool. Perhaps a small fuel cell like the one from Intelligent Energy could help with range and power. A BRS would help with safety.

As a helicopter pilot, I have these comments: 1. It look incredibly easy to fly... huge plus. 2. The amount of rotors tells me that more can go wrong...and unless this aircraft can autorotate, it will never get FAA certification. From my experience and knowledge of autorotation, I see why it wouldn't - if they can disengage the rotors from the electric mechanism quickly and allow them to freely spin with controls to increase drag. I'm sure they have that all figured out, of course. 3. Being electric is amazing... I've only flown piston-driven helicopters - and mixtures, fuel issues, magneto issues... all that goes out the window.
Conclusion: Sweet.

I would like to see a test flight (unmanned) where they shall out two or more motors.