Ask HN: How do you bill your clients? (for your app)

We are building a software service that will be paid for based on a monthly subscription model. We are not really interested in reinventing any wheels and writing a billing system, and are interested in what you all are doing for your applications.

Do you create bridges to other third party billing system? If so, which ones do you recommend to integrate with (we are a ruby on rails application).

If you think its a bad idea to use a pre-existing billing system, could you give reasons why you feel that way?

Thanks for the advice!