Which payment service providers have the most developer-friendly APIs to integrate against?

I've been doing some payment-integration work lately, and the PSPs I've been working with have been painful to integrate against:

• the back-end interfaces seem be stuck in the 90s in terms of usability and technology (frames! Argh!)
• the APIs are badly documented - finding a list of example responses, or a complete list of errors is hard or not present
• support is not particularly responsive or high-level-of-technical-expertise

Surely there are PSPs that are technically up-to-date with friendlier, well-designed interfaces and APIs?

Which ones have people had good integration experiences with?

(Am asking purely开发者_运维知识库 from a technical level; commercials are out of scope (I'd still be curious, but that's not the point of the question).)

Update: I'm based in the UK, but working for a company whose website operates one instance in each of several countries throughout Europe.

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I work at a company called http://checkissuing.com ...thats exactly what we do.

Per your request, we have a very robust SOAP/XML AP, let me know if you would like to look at the specs

We have been around since 2005 and are experts in the field...let me know if you have any questions or want to talk :)