Not only is it very well done and extremely "educational" but it's also fun. Unlike other games of the same genre you're not judged by your speed and accuracy. In Typeracer it's all about speed. Your mistakes is your problem since they stop your speed. It's hard to explain but it just works great. Play it and you'll see.
Having played this I realise that my spelling is absolutely terrible. I've grown to depend on the spellchecker in Firefox and when I'm doing programming I don't have to spell particularly much because I use auto-complete. At the moment I average around 55 words per minute which means I win quite a few races but still far far away from the high scores which start on 100+. Gotto to practice more!