I will say this - and I mean this in the most respectful way possible: it is unlikely I would encourage us to switch to you folks until (frustratingly!) you all are more established!
[Also, I don't mean to single you out specifically: I'm talking about any company who wants to be an integral part of our infrastructure, but the topic at hand happens to be payments.]
I have personally been burned by choosing a neato new startup for critical parts of our infrastructure. Burned in the sense that these startups change their model and pivot quickly, and since we have made them an integral part of our infrastructure, it is frustrating and difficult to change. (Dotcloud, for example, got rid of their free sandboxed tier this past year. This was a great move on their part, but also left a sour taste in my mouth and unexpectedly increased our expenses.) We also get burned when a new company has bugs - because while I personally might understand, our customers are the ones who are affected. It also called into question my ability to evaluate different solutions, and forced me to realize that, for non-personal projects, it is important to use something established.
This is somewhat ironic, since we ourselves are a startup - so I feel the pain of finding customers (or even developers!) every day!