What sites use tastypie as their webservice API framework? How does tastypie compare to piston or django-rest-framework with regards to long term maintainability?

I would advice to build a proof of concept in both django-rest-framework and tastypie. They both have features you might want and limitation you don't want. Build an API with them and then try to do something very silly (for instance: Have a model with a foreign key to another model which has a slug. Then try to implement an API in which you can specify that relation using a slug without exposing the related object) That's really the only way to discover if the framework is suitable for your project or not. Also keep in mind that both those frameworks are tied close to the models, which in turn exposes a lot of information about your code structure.