Im a 1500 elo jungler who climbed from b4-S1 by playing lee sin/kha/vi. I have 141 games on lee sin and my win rate is around 55%.

Lee sin is a high Risk-Reward Champion and patch 4.13 has hurt him a bit but he is still one of the best junglers with still some of the most intense early game pressure in the game if played right.

DONT PLAY LEE SIN IF
1. You don't have time/patience to learn him
2. You don't know how to use early game champs
3. You don't have fast enough fingers/good enough ping/good enough fps to insec
4. You are scared of lee sins late game

PROS
1. Extremely high DMG early game
2. Extremely high pressure early game
3. Can gank from anywhere
4. Great tower diver
5. Great for SoloQ
6. Not item dependent; all you need is a sightstone and some HP to be relevant
7. High reward
Cons
1. High risk
2. Late game is horrid
3. Takes a long time to play lee sin at a competitive level
4. Takes a while to learn how to insec

Get mobis before you finish Spirit of the Elder Lizard
Why?
because you want to push your early game as much as possible and the best way to do that is to get early mobis.
Make sure u keep ur trinket until u get sight stone and then change it out for sweeper\
ALWAYS GET SIGHSTONE
Replace mobis with Merc treads late game.

Go into a normal/Co op vs ai/ Custom with bots
to practice your insec kick. Once you think you can pull it off in a real game, go try it in normal a couple of times. its ok to mess up a couple times but just keep trying to do it. You'll get it. you need to.

Go Blue - Wraiths - Red
You need the red slow for ganks
you clear wraiths really well as lee sin bc of your e.
After your lvl 3 from those camps focus on ganking. force ganks. even if u don't get kills get summoners or just have presence.
its ok to farm but you don't scale well at all so its always best to focus on ganking hard.
Use your passive(as buff when u use a spell) to clear quicker

if your ganking, use your trinket-w combo to close distance to get e before you q so its almost guaranteed flash or hit q.
If your lvl 6 use ward to close distance then e, then flash kick, or q-q-w-r (insec kick)