Well my experience with braintree is that if your business model is innovative enough they won´t take the risk to process your payments (told personally by a braintree representative in a phone conversation, after a brief explanation of our business model). At least they are very kind, and they´ll tell you before hand, so you don´t have to waste time with all the legal paraphernalia.
A pity because the API seems really fast to implement and well thought.
Respecting Paymill... well we are not going to put data on the hands of the Samwer brothers right away(just in case...).
The API does work like a charm. If history can be a reference, Paymill is going to get bought by one of the big players who want a quick start in the European market (Stripe comes to mind). But I do get your reluctance.
By now we are working with Paypal to show the concept (Not an API you would recommend dough...). But we are planing to create a Ltd. in the states to work with Stripe. It will also make it easier to close a round with american VCs. We are based in Spain and the Spanish VCs are not what you could call that "venturous". They are more likely to invest in "me too" startups, or proven models.
Respecting Paymill... well we are not going to put data on the hands of the Samwer brothers right away(just in case...).
edit: typo.