This post was inspired by one about English vocabulary. I too noticed an improvement, but not with my vocabulary (though I must admit the wanikani team has some rather creative writers on hand)
Anyway, What I really noticed while using this app was that my SPELLING ability was actually getting better. In an age of automatic correction, regardless of how many times you’ve ever typed out “separate” as “seperate,” “recommend” as “reccommend,” or “vacuum” as “vaccum,” you somehow never manage to actually learn how to do it for real. Now I know WaniKani accepts these alternatives since they closely resemble the original, but since I’m a perfectionist I’ve really drilled in all the common mistakes I’ve ever made. Again, not that this is particularly useful since everything online gets auto-corrected anyway–I just thought it was a neat little bonus, at least in my own case.