The efficency is said the be 80%, just like any other pumped storage system. It just uses considerably less space than a water reservoir (actually close to none, as the top of the cylinder remains untouched) and is scalable by adjusting the radius.

I guess the main problem is to keep the water confined (200bar of pressure) while keeping the mass movable. Maybe someday they build a small testing facility, so we know for sure.

The point is to have a way to store the excess energy we generate at off peak times like the middle of the night.