I haven’t, though I was working at W|A when it was being designed. There are actually a few APIs available. It pretty much returns a structured version of the results on the site, which are often inconsistent even for similar queries. I think it’s worthwhile to implement around those inconsistencies, but depending on your application you may not.
I am having some difficulty getting the PHP API to work at all for me. I found at least one error in the original example code already, and so was looking for someone who had a working version of the PHP binding I could look at to see where mine is messed up.