Lee Sin is a really strong jungler at the moment, with extremely good early game.
He has good clear and good ganks, mobility, 1v1, outplay potential, good teamfight.
Could you ask for more? :D
It is an insanly rewarding champion I really recommend playing it.

Make sure to use his passive well, always do 1-2 autos between spells since they are faster and give you energy back.
When you are ganking dont throw your Q right away it's easy to dodge if thrown from max range. Just walk up and wardjump on their face. If they try to sidestep just use ur E slow and auto them down and throw your Q when they the least expect it.
You can get a guaranteed Q with your ultimate. How to do it? Just R the target and don't move your cursor, press Q, done :D Lee Sin has a lot of burst this combo alone will probably oneshot squishies.
You can't Q flash to cancel the animation of your Q. But you can make a lot of cool combos using this. If you press Q flash the Q will be thrown from where you flashed. Use this to do some unexpected Q ult plays.

Make sure to upvote if I helped you out it would mean a lot to me! :D