Lee sin is a very mechanically intensive champion that require time and determination to master. Overall he is a much better jungler than anything else, but lee mid allows you to abuse lee sin's mid game damage and then transition into more of a fighter instead of assassin late game. Above gold, Lee mid is not a very good pick unless you know are very mechanically advanced with Lee. You will have much more success in climbing if you play other AD assassins mid, like Zed and Talon. However, if you are having fun go ahead and learn all there is to it. I usually just play Lee mid as a pick to have fun with in normals nowadays.