I stress-purchased some digital brushes the other day, and so far I have only been regretting that decision in terms of having less money -- having the brushes has been fantastic.
I bought the brushes from Kyle T Webster's Gumroad store. I got the cross-hatch set, the screentone set, the animator pencil and the Mr. Natural brush. So far, my favorite combination is definitely the screentone set with the Mr. Natural brush. I've done portraits like the one to the right (from my favorite picture of me) of like half my friends in the last few days.