About cleanliness, taxis and rental cars are clean because there is someone there to clean it between each customer. That doesn't work for carsharing, where cars go directly from a customer to the next.
My experience with these systems is that you don't find anything really dirty in the cars, but you tend to find empty bottles and things like that in them. Not a deal breaker if you just want to get somewhere, but not very family-friendly either.
Car sharing services in Japan ask you immediately (via the navi) what the state of the car is. I imagine it negatively impacts the previous rider in some way. A simple social pressure like that can work wonders.
My experience with these systems is that you don't find anything really dirty in the cars, but you tend to find empty bottles and things like that in them. Not a deal breaker if you just want to get somewhere, but not very family-friendly either.