+ Strong Early Game
+ Strong Burst
+ Strong Ganking
+ High Mobility (Good Follow Up + Escape)
+ Works good in nearly every teamcomp
+ Playmaker (especially good for SoloQ)
- Weak Late Game (scales really bad)
- Hard to master
- Not much AoE
- No farming machine

Beeing useful in Late Game

Difficult insec/lantyr-moves
If you are good enough with Lee Sin to do a Flash/Ward Jump + Dragon's Rage (insec) you will always be dangerous.
Teamfights:
Use your high mobility combined with Dragon's Rage to peel for your own carries or bring the enemy carries in trouble.
Warding:
Don't forget that you have Tracker's Knife. Use this to ward important places on the map. You probably aren't such an easy target as your support.

In the beginning you should always make 2 autoattacks before you cast your next spell to use Flurry as efficient as possible.

Try to gank as much possible to get a use from your good early game stats.

Don't forget that your Resonating Strike is an Execute. So you should deal as much damage as possible before you (re-)acitivate it. You can use this also for securing jungle buffs, Dragon or Baron Nashor if you use a combination of Sonic Wave, Smite and Resonating Strike.

If you use Dragon's Rage on someone he will be unable to move while beeing in mid-air. Because of this you should try to hit your Sonic Wave right after using Dragon's Rage - you can't miss it then.

Lee Sin is a great champion for counterjungling, because of his high mobility and burst potential. If your mid/top/bot is able pressure their lanes you could go for some counterjungling action. Be atleast sure to put some deep wards in the enemy jungle to get as much intel as possible.

Kill the crab in combination with Iron Will to heal up before ganking.