... easy, benchmarking your own algos against the prediction API on large-scale data sets. I presume that's what 90% of the people who received their account during testing did anyway :-D I did 15k requests in about 10mins at times.
Wanted to use it with Google Analytics for a kind of recommendation engine of the form "users that liked this page also liked...". But they never approved my request and a 60k request limit won't cut it either.
I'm using it in an app, since my request did get approved, but it's very rudimentary at the moment as I am still exploring it. I'll be sure to report back to HN at the point it is in full use.
To me a big weakness of the prediction API when I checked it out was that it cannot handle (i) missing values or (ii) sparse data formats. Both are very common in real life data.