I've heard of for some time now, but never actually tried anything on the site. Today I tried a course that teaches the basics of R, a statistics programming language akin to Matlab, and I thought the site was great. I suspect it all depends on the quality of the course, but at least this one was very nice. You can see my "report card" here, although I doubt I am going to visit the site very often. However, for beginners or people who quickly want to "get" something, it is a good place to start, as it gives one a "hands on" experience, like actually coding to get results, but in a carefully explained step by step tutorial format.