This is a really fun and inventive game. I'd say it's 90% about concentration and only 10% about typing, as all of my mistakes come from going "too fast" and not leaving enough keys up.
I think it'd be interesting to create a "survival mode" where order doesn't matter, but the slots fill up super quickly and you have to keep them clear them before the entire fills up (and the game ends).
My current high score is 23 — take that @math2001
Yeah, it really has all the makings of an addictive one!
I myself find it challenging and addictive :)