The way that I use this build is to gain levels and money by jungling, and then ganking the lanes. In the last game I played, I often went 2v1 and consistently won. But this can still be played two ways: After about level 10 or so, you can begin pushing a lane, otherwise stick to ganking. But Lee Sin is a lot of fun to play, and with a bit of practice, he makes for a very strong competitor.


Edits:
4/10/11 Changed starting items from Cloth Armor and 5 Health Potions to Vampiric Scepter.

At the start of the round, I actually grab Vampiric Scepter. It works better than one might think, and if you're low on health, farm some creeps between jungling. When I tried this I was skeptical at first, but I found it really good for the build, and worked well. From their, I grab Madred's Razors as soon as possible for the bonus damage to creatures. I upgrade to Wriggle's Lantern for the Lifesteal and increased chance of Bonus Damage. Mercury treads come after, because I like their ability for ganking, so you can more easily escape. However, you could easily change these out with whatever boots you find appropriate for the round. From there, I get The Brutalizer, because it's rather cheap, and reduced ability cooldown and armor penetration are nice for both creatures and summoners. I like getting Sunfire Aegis around this time, because I often find myself forced into a lane, and it's useful to have for both health regeneration and pressing the lane. However, this item can also be changed based on the situation. From there, I use The Brutalizer to upgrade to Youmuu's Ghostblade, because the abilities and the effects are very useful. For it's active ability, I like using it right at the start of a battle, and it's nice to have before a gank. From there I like using Last Whisper just for the added effects. Bloodthirster is a nice way to top it off, but you can really put anything you want there.

SITUATIONAL ITEMS:
Sunfire Aegis can be replaced with Thornmail against a strongly AD dps team. It can also be changed with Guardian Angel if you need both armor and mag. resist.

For the last item, that's really up to you. Very rearly have I been able to finish the build, seeing as it ends before that often enough. But some items to consider instead are Black Cleaver due to its increased attack speed, Frozen Heart for the armor and Reduced ability cooldowns and reduced attack speed of nearby enemies.


ANOTHER OPTION:
For when you find yourself doing a lot of ganking, or maybe even needing to push a lane, you could try not getting Wriggle's Lantern and instead get Madred's Bloodrazor before Sunfire Aegis. You have a lot of time between Mercury's Treads and Madred's Bloodrazor, so you might want to have yourself already in a better situation, where you can either gank consistently or jungle without much worry.

I always like starting with Safeguard / Iron Will because of Iron Will's increased Life Steal, Armor, and Spell vamp. From there, I go to Tempest / Cripple for the AoE effect. NOTE: I don't follow up with Cripple unless against champions, or when you need something slowed down. From there, I go to Sonic Wave / Resonating Strike So as to have all three basic abilities. From there, I want to up Safeguard / Iron Will ASAP for Iron Will's abilities, Tempest / Cripple next for the AoE, which is also useful for Iron Will. I max out Sonic Wave / Resonating Strike last because, frankly, past the first level, I don't see as much use to it besides chasing.

I like using Ghost because of the versatility of the move.
I use Smite because, being a jungler, it's very much a necessity.

Rather then Ghost, you can use whatever you want to (except for maybe clarity haha)

PROS:
Deceptively durable with the right moves
Strong jungler
High survivability

CONS:
Somewhat squishy
Has a hard time 2v1
Difficult to kite

For jungling, I collect my Cloth Armor and 5 Health Potion and book in on over to the wreaths. All jungle creatures appear at exactly 1:40, so be prepared. Take out the Blue Wraith using Smite and the rest with attacks and Safeguard/Iron Will. From there, head over to the wolves, do the same thing. I've tried getting the blue buff next on several occasions, and it's only worked once. I suggest you move on to the twin golems from there. At this point, recall and use whatever money you've got to buy what you can. From here, collect the red and blue buffs. This is usually when I start ganking, and it can work well, but don't go kamikaze style, you don't want to give the enemy your brand-spanking-new buffs, do you? At around level 7, I take on the dragon, and at around 14 or 15, with I friend, I might take on Baron. After that, it's up to you want you would like to do. As a jungler, I suggest you constantly collect buffs, and prevent the enemy team from doing so. Also, the jungle is a nice place for 1v1s, so that's nice.

Thanks for reading! Please, place any suggestions you have into the comments section, and I'd be glad to add it to my guide. This is my first one, so please don't be too harsh; I'm fragile :)