Re. Service contract/rental model, the economics of providing that service are extremely challenging. People who need this product have very little money to spare. The price would need to be set low enough, and in those price ranges the profit margin basically disappears.

For other interesting companies doing this with these cost and pricing issues in mind see:

http://www.xrunner-venture.com/

http://resourcesanitation.com/

Edit: full disclosure, second link is my brother's venture in the space. I've heard a lot about toilets over dinner for many years :)