Pros
Good Survival
Good damage
Ability to escape situations well
Excellent harass

Cons
lvl 1 Lee Sin is utterly useless

The great thing about Lee Sin is his amazing harassment ability. After you get past his lvl 1 uselessness you will be able to be one of the most annoying harassers in the game.

To harass all you have to do is sonic wave/resonating strike in then safeguard out, making sure that you have a friendly unit close enough to safeguard away each time.

This harassment even allows you to harass them at the tower if you safeguard out fast enough most of the damage from the tower is taken by your safeguard shield making lee sin one of my favorite harassers.

Later on when you have a couple lvls into safeguard and wriggles lantern you can pretty much heal half your life off one creep wave with a safeguard/iron will.

Runes
Armor pen quints and marks will help you put out a lot of damage with Lee. The dodge runes work very well especially combined with ninja tabei and the nimbleness mastery to help your survival.
and the magic defense glyphs help you in those battles against mages.

Items
The goal of this build is to be a tankish hero with nice burst damage. The main focus being to survive. With that in mind first item you go for is cloth armor with 3 health pots for early defense and health pots to stay in the lane longer.

First time you get a chance to go back you will want to get your tier two boots the Ninja tabei for the extra dodge defense and movement.

Second item you will want is wriggles lantern. This item is a great item to have as lee giving you life-gain armor damage, and its activated and passive are some of the best in the game. being able to throw up a ward every 3 minutes helps your survival immensely stopping potential ganks. While the chance to do 500 damage to a creep helps you to farm easily throughout the game.

After the lantern you will go thru the prerequisite for trinity force. once its completed giving you boost to nearly everything but the best part being the help in burst damage it will give you. after resonating striking into an enemy you can hit them for 150% of your damage dealing a good chunk of damage.

The next item some people may argue against because you already have the % chance to slow enemies with the trinity force but by getting frozen mallet you have a guaranteed slow as well as an extra 700 life witch will allow you to survive easier as well as chase down kills for you and your team very easily with your sonic wave and slow on hit nothing will get away.

At this point the game is more than likely over but if you are still going your next two items are dependent on what you want really if you want more survival guardian angel might be good for your next item if you want more damage bloodthirster is a great item but at this point its really up to you.