I have not used it, but I have seen them (or someone like them) in San Francisco, and for places like that- if I lived in a truly urban environment - I think the concept is brilliant, and I would be sorely tempted to participate.
Corollary:
I have to have a big car for clients, jobsites, taking folks to court, etc., but I hate driving a big car. In our "space" in the legal world, nearly everyone has an SUV of some kind or another, lots of Suburbans and Escalades. What a waste! My compromise is a Caddy deVille, which has lots of room and (remarkably enough) is not too bad on the gas (solid mid-twenties, highway mileage).
My 'druthers would be to drive something smaller and sportier (Audi A3, A4, 3-series BMW, something like that?) and more economical, and have a truck available on an "only when I need it" basis. Could rent, but uncertain availability, ridiculous cost for rental of vehicles like that.
May just buy a 5-year-old one and keep it in the parking garage for firm use...