I had a freelance project that used Interactive Brokers , which provided both their own API and FIX-based. The patented API is somewhat painful to use (and they probably can't change it much for compatibility), but you can easily code the "translation layer" to make it much nicer. I have not tried the FIX-based API, so there are no comments there.
And yes, I opened the question again. I think this is a fair question.