Like most offal, kidneys have a flavor not loved by much of the American palette. I have not done kidneys in a long time, mostly due to cost. If you like liver, heart, etc. kidneys will fit right in the mix. I remember slicing them really thin (less than 1/4") and frying them in a really hot pan (do this outside!!) for 30 seconds per side. I do liver the same way.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS