So, I have been playing Lee Sin since he got released and REALLY enjoy him and he's amazing if played well
I've often heard that I'm a really good Lee Sin and have often been asked how I play him and what my build is (For example, one of them is now kind of my "pupil" :D)

Pros
- Deals good damage
- Can jungle well
- Is pretty tanky
- Can chase well
- Good ganks
- Can save the WHOLE team
- Energy user

Cons
- Most of the time first focus
- Q is a skill shot
- Sometimes there is no ally close when needed


Change log:
-16 July 2011 11:49: Added Jungling and Ganking
-18 July 2011 10:01: Runes and Masteries for Jungle Lee Sin
-18 July 2011 14:25: Added a LoLReplay jungle file

I use the following
Greater Mark of Desolationx9
x9
x9
Greater Quintessence of Desolationx2
x1

Armor Penetration: Lee Sin deals mainly Physical damage and helps with last-hitting
Dodge: Dodge is insane on any champion, flat armor can be used as well
Attack Speed: I don't have any other Glyphs, except AP xD But Magic Resist is a good choice as well
(And I have only 2 Quints of Desolation, and use that last one for Jax as well)

I run basic 21/9/0 masteries, taking Improved Exhaust and Ignite and Nimbleness

I alway use
Makes sure no-one ever gets away
To finish off those annoying champs that always get away (or if you're really fed, u could ignite someone > E > Q > R and focus other champions while he is dieing xD)

Great as well
Great for chasing (but you have Q and W as well, or E to make sure they don't get out of range)
Normally you never should take this, but it's great in laning phase and could save that one teammate from Karthus.
If you're jungling, you take it. You do. If you don't, I'll come to your house and kil u.
Also insane to use in combo with your ulti. Just Flash in, Dragon's Rage enemy into your team and dash back with Safeguard

Less, but still viable
For those lazy people (or if u go solo top)
If the enemy team is something like Amumu, Alistar, Blitzcrank, Taric and Sion
If no-one else is taking it and the enemy has pushers like Mordekaiser, Twisted Fate or Sivir
Actually, it's great to make sure you don't get ganked, to check on the enemy jungler, but the support should take it

No-go
Never take it, unless you're trolling.
See above
You have Energy...unless you're playing as a support and want to help your lanemate ?

Lee Sin is an amazing champ, with 7 skills (just like Nidalee, but she has a secret 8th one)

Flurry(Innate): After Lee Sin uses an ability, his next 2 basic attacks gain 50% Attack Speed and return 15 energy each.
This is a great passive, make sure to attack at least once after a spell cast

Sonic Wave(Active): Lee Sin projects a discordant wave of sound to reveal his enemies for 3 seconds, dealing physical damage to the first enemy it encounters. If Sonic Wave hits, Lee Sin can cast Resonating Strike for the next 3 seconds.
50 / 80 / 110 / 140 / 170 (+1.0 per bonus attack damage)
Resonating Strike(Active): Lee Sin dashes to the enemy hit by Sonic Wave, dealing flat damage plus 10% of their missing health as physical damage.
50 / 80 / 110 / 140 / 170 (+1.0 per bonus attack damage)
One of the best harassing, iniating and chasing skills in the game ! Deals good damage and dash ! In lane use it to harass, when chasing make sure to hit the enemy with hit or use it to escape with jungle creeps ! I max this first

Safeguard(Active): Lee Sin rushes towards a target ally, shielding them both from damage for the next 5 seconds. After using Safeguard, Lee Sin can cast Iron Will for the next 3 seconds.
40 / 80 / 120 / 160 / 200 (+0.8 per ability power)
Iron Will(Active): Lee Sin's intense training allows him to thrive in battle. For 5 seconds, Lee Sin gains lifesteal, spell vamp, and armor.
5/ 10 / 15 / 20 / 25 %
10 / 15 / 20 / 25 / 30 armor
Safeguard allows u to save a teammate, harass well and maybe survive a turret hit or make sure your mate doesn't die. Iron Will is a bit more important, as it makes u stronger in a 1v1 against a melee DPS (along with Cripple), makes your jungle easier and allows long laning. I max this last, but take ~2 points in it for the survability

Tempest(Active): Lee Sin smashes the ground sending out a shockwave that deals magic damage and reveals enemy units hit for 4 seconds. If Tempest hits an enemy, Lee Sin can cast Cripple for the next 3 seconds.
60 / 95 / 130 / 165 / 200 (+1.0 per bonus attack damage)
Cripple(Active): Lee Sin cripples nearby enemies revealed by Tempest, reducing their movement and attack speed for 4 seconds. Movement and attack speed recover gradually over the duration.
30 / 37.5 / 45 / 52.5 / 60 %
Tempest is quite unique, as it scales with AD but deals magic damage. The damage isn't that bad and it allows u to farm well as well as reveal champions like Akali and Shaco and Teemo's Shrooms. Cripple is great as well, as it makes ganking easier and has quite a slow (60%) but is countered hard by Mercury's Treads. I max Tempest/Cripple second

Dragon's Rage((Active): Lee Sin performs a powerful roundhouse kick launching his target back, dealing physical damage to the target and any enemies they collide with. Enemies the target collides with are knocked into the air for a short duration.
200 / 400 / 600 (+2.0 per bonus attack damage)
Lee Sin's ulti is insane, with a AoE knock-up, quite some damage, nice scaling, just a minute cooldown and great for saving yourself/teammates. Take this skill at 6/11/16

So I go Q E W Q Q R Q W Q E R E E E Q R Q Q

Start with Long Sword and Health Potion
After you got ~900 or ~1150 go back to finish Phage (and Boots)
Next I like to go around ganking/"jungling" and farm enough for Berserker's Greaves or Mercury's Treads and Frozen Mallet

The next item is usually depending on how good I'm doing. If not going well I take a Youmuu's Ghostblade for easy turret pushing (with Flurry) and escaping. If doing a rather good job, I go for Infinity Edge for quite some damage and some crit.

So core:

/
/

The following 3 items are completely depending on the enemy team:

Heavy AD:

Heavy AP:

Balanced:

No team composition at all (stuff like 3 supports and 2 feeders etc.):

I like Randuin's Omen for its active, which is both offensive and defensive ( Frozen Mallet + Lizard buff + Cripple + Randuin's Omen = Quite some slow)
Atma's Impaler is a must, since it gives crit, some armor AND 2% of your hp as attack damage, which is ~30-60 or so

Runes and Masteries
I use the same runes as when laning, but I go 2/18/10 masteries, because I want to jungle for as long as possible and be on 100% health when ganking. Taking improved smite and exhaust, Nimbleness, armor, negate damage from monsters and bonus health.

I take Exhaust and Smite when jungling, because Smite is a must, and I love Exhaust

Jungling
Lee Sin is an epic jungler. For me, he is the best. He can stay on full health from ~lvl 3, his ganks are amazing both pre- and post-lvl 6 . He doesn't need his blue buff, so he can donate it (I still take it, the cooldown reduction really helps with jungling)


When jungling I start with Boots + 3 health pots (or 4, if u want to wait) and skill W first. You could start with Cloth armor + 5, but I don't like Wriggle's Lantern, or any other item it builds to.

I head for blue with someone to pull it, make sure to smite with Iron Will active, so you heal ~20 hp
After every spellcast make sure to hit the minion at least once. Put one point in E

Next up is wolves camp, W > 1 hit > W > 1 hit > E > 1 hit > E > 1 hit etc.

Head for wraiths next. Same as with wolves, put one point in W again

Go for the mini-golems next. Usually I can have them, but sometimes (I don't know why) I have to smite them, because I get dangerously low

Next is red (if I'm sure I can survive it, else I wait ~20 seconds and go Wolves > Wraiths > Mini-golems > Red). Same as with all others.

Afterwards I just keep jungling around, getting some minions, stealing creeps from the enemy jungler, gank him etc.

Ganking
Lee Sin is a great ganker, BUT you need to hit your Q to be succesfull. When ganking hit your Q, chase him, E > E > Exhaust and keep attacking.
Post lvl 6 you walk in front of him and knock him away from his turret

When ganking ask yourself a few things first
- Are there not too many minions ?(on lower levels they're DANGEROUS)
- Is the lane you're ganking on good health ?
- Is the person you're ganking on low health, and can u gank him ? (People like Janna are very hard to gank, because knock-up, knock-away, slow, flash)
- Is the lane not warded ?

When you answer Yes to all of these things, you could gank.

Here's a LoLReplay file, with me jungling as Lee Sin (I'm afk at some moment, phone rang and got a bit too chatty (A))
http://www.mediafire.com/?61bmy9sfxuzmm6z

Champions you like to play against:

- Ashe: She's squishy, an auto-attacker and isn't that fast, so easy target
- Akali: Lee Sin is the ultimate counter to Akali. She won't be able to hide in her shroud (because of Tempest) or dash to a minion (because you'll chase her with Sonic Wave)
- Evelynn/ Twitch: They're completely stealth based, so you'll hunt them down and kill them with ease (but you'll probably never see one xD)
- Teemo: Just go hunting for his shrooms xD
- Shaco: You're an anti-melee DPS (because of your Iron Will and Cripple) and anti-stealth, and luckily Shaco is both Melee DPS and stealth :D When he jumps over a wall, jump after him with Q.
- Gangplank, Jarvan IV, Jax, Master Yi, Nocturne: All of them are melee DPS and need to get close to damage you, but you have Cripple to make their attacks useless, Iron Will to decrease the damage taken and heal up and Dragon's Rage to escape if they're doing too much damage. Jax is dodge-dependant, but you won't auto-attack him that much at all.
- Janna, Sona and Soraka: Squishy supports, just reach them and they will fall easily

Farming
Shouldnt be hard, just press E and use Flurry to quickly get rid of the remaining ones

Escaping/Chasing
Some days ago I had a funny game, with quite some amazing actions made by me:
- When getting chased (I was being chased by ~4 champions) u can use Q > Q to rush to an enemy minion (in the jungle or lane) and instantly follow it by W to an allied minion. This can be quite great. If they get too close, try to aim your R well and hit as many champions as possible
- When chasing u can use the same technique, by jumping to an allied minion/ward and Q to an enemy minion/champion.

So, that's that for this guide, if u have any suggestions, comments or tips don't be afraid to comment :)
You can add me ingame: Roxoz
And if you want, you could join our Dutch LoL community forum :D http://forum.ladineko.nl (it's actually my friends site, but he dedicated the forum to LoL :D)