I've lived in several cities in europe and i'm not actually sure i've ever seen anybody using them. Yet there seems to be tons of competing companies putting out bikes in the street.
On top of that, they don't even seem to bother marketing or anything. The first I'll hear of a new company is seeing a new colour of bike.
The idea is sound in practise, but people seem to place great value owning their own transportation and the flexibility it affords them.