So far, so good. It's quite engaging, fun, and gives me a nice workout and practice session. I play a few rounds then I'm done for a while, but can keep coming back to it. I'm not sure how many nights I've completed. Some where around ten, I suppose.
I can't say it's been very challenging. I think I've only taken damage once. Though, I am a fairly proficient typist. It seems to be gradually scaling up.
My only complaints are that the menus just look rather ugly, honestly. Big, clunky, seeming like they're in a resolution entirely too small for them. Also, the auto-save feature seems like it occurs when a might is completed, but I can't quit the game until I start the next night after I've purchased upgrades. Yet, upon continuing my progress later, I'm forced to repurchase my upgrades again before beginning. Unless I'm mistaken on that.
Neither are big issues. Most of the time I'm just pounding away, and can't even really tell what's going on, what I'm killing, etc., but it is quite satisfying. As far as typing games/tutors I've seen, I'd say you've got one of the best here. I believe the first I ever saw and played was "Type Trek".