Ask HN: Using the Pay What You Want Model

I'm in the process of building a Rails app (http://progrss.me), and have been putting some thought into different revenue models to (at least) cover the running costs.

One alternative that I'm considering is using a "pay what you want" model with different tiers (including $0). The amount you choose to pay will have no effect on your account, but I'm thinking of having something resembling a Kickstarter for features.

Whenever the total contributions reach a set amount, I would implement a new predetermined feature into the service. Hopefully, this would give the users enough of an incentive to contribute to the project and keep it running.

I would like to hear if anyone here has had any success with the pay what you want model, or if you'd recommend other alternative revenue models instead.