There are several commercial apps available for Fusion that were written using the current API so I think you could say it's ready for productive use. However, it does depend on what you need to do with it. The API doesn't provide complete coverage of Fusion's functionality so if you need access to capabilities that aren't exposed by the API then it's not ready for you. If you're doing modeling or querying existing models, there's a good chance the functionality is there. If you're doing other things, like simulation or drawings, there's not any support for them.