We applied and simply got a rejection email with little to no explanation of why we got rejected which is disappointing. I'm going to try and open up communications with them again to see if we can reapply now that we are fully trading (unfortunately through paypal)

  The evaluation of new applications in order to accept credit card payments is subject
  to strict compliance rules and risk regulations in Germany. Our current acquiring 
  bank has declined your contract for some reasons with regard to their risk and compliance 
  management guidelines. We do not receive any detailed reasons from the bank.
  This is why we cannot give you a better explanation. Unfortunately, 
  Paymill does not have an influence on that decision.

We began to look at them, unfortunately while the API was straight forward the documentation was simply wrong in many places and contradictory which was a tad frustrating. The biggest issue for us was they simply couldn't answer even the most basic question like their current PCI-DSS status, information about their network or datacentre setup (or even where it was hosted) in effect we couldn't complete any PCI-DSS assessment docs. Needless to say, we simply did not feel confident handing over our companies account and other details they were asking to go live with. Let alone let them handle our payments. This was a few months ago, and they were having teething issues which may have now been resolved I suspect given how common such questions should be they now have the answers in place we may well relook at them in a couple of years time if they are still in the marketplace.

I've been using it since September with zero problems so far. They answer their phones, which is more than I can say about anyone else in this space. I wrote a python wrapper for their API as well. Everything's been smooth. If you have any issues with them, call them, and in my experience things get fixed instantly.

I'm using it and I wrote the Ruby wrapper (https://github.com/dkd/paymill-ruby).

I like their service and they're nice guys.

Yep, we are using it and never had a problem so far. I encountered one bug in the Magento Extension but they fixed it in a few days.

We will be using Paymill for our upcoming project. We haven't yet gone live, but the api was very easy to use and it seems to work very well. One downside for us is that they only accept Visas and Mastercards.