It reaaly gets difficult with the all keys and both hands mixes... thought I'd get through this all until I hit those, but maybe best save them for some other time.
It is great exercise, but also a little unnatural when you actually have to hit individual keys at individual times, rather than type letters as soon as you see them, or type out complete words.
Mastering this seems like a step beyond mastering typing! But great game.
-cd-