I used it infrequently as a consultant; my company actually introduced me to the service -- but we used it to supplement traditional rentals, not replace them. Got to run out to play pet engineer for a scoping meeting? Grab a zipcar. Stuck in the ass end of the midwest for a two week engagement? Traditional rental.

I don't think they are leaving it on the table, I think their schedule of fees means they are carving out a spot that the rental companies totally ignored.