One minor but very nice aspect of the UX is that I was able to click the link and immediately try it out. I wasn't even planning to really use it but ended up completing a round. My only complaint is that the drag and drop is kind of annoying as the default selection process, clicking would feel more natural.
For comparison I tried doing the same with Duolingo and the UX is much, much worse. After multiple clicks and two noticeably long loading screens the first question I got was "How did you hear about Duolingo?" followed by a question about why I'm using the product. Blech! I wanted to try out the product, not help their marketing department.