I'd personally never heard of them before you asked this question. But I have to say, based on the website, I'd probably be unlikely to enroll. This is from their website FAQ:
2. Do students who sign up for membership and classes need to be identified officially?
No. Many high potential students benefit from our academically rigorous curriculum offered online through our various classes. Students who ponder, wonder, and want to investigate more will be stimulated with these classes. We have found that there is a spillover effect in that often students who are not gifted begin to piggy back on ideas and blossom into active scholars. Students must be serious about learning.
While I'm sure "high potential"/"not gifted" students *do* benefit from the classes, my goal when I'm seeking gifted enrichment is to give my child the best chance to benefit with similarly gifted peers. In a class like this I'd worry that a gifted child might be frustrated by a slower pace, a lack of depth and/or teaching styles that work with highly motivated NT students versus gifted kids. We have enough of that in day-to-day; no need to pay extra.