I've used it. Its actually one of the nicer APIs, but since you can't republish market data from it, you're stuck with only giving derivations and projections. That will satisfy some people, but there's definitely a group of traders who always want to compare your results to the raw data points on site and not jump between their own data source and your algos output.

Hmm... did not know that you couldn't republish market data, I hadn't yet looked over the developer agreement I was only given access to it this past week...still not sure how I'll use it