I've played him exclusively in the jungle for quite a while now (as part of my large pool of junglers, lol); he's definitely a strong choice. Like most junglers, he's not that useful if you fall behind, but he's a great ganker and early duelist if you play your passive effectively. I'll give more detailed suggestions if you want them, but here's just a few: build the warriors enchant and include Randuins Omen in your build (you will need some survivability); you should be looking to gank bot lane as soon as you hit level 6 (it greatly helps if you have an ashe and/or leona on your team to help set it up); and lastly, Pantheon counters Lee Sin at every stage of the game. Unless you and he have a huge skill and he outplays you super hard, he shouldn't be able to fight you, ever. Strongly consider picking him if the enemy takes Lee