Renekton: critical win.

Crit chance: 80. (not to mention crits are at 260% b/c Infin Edge and Masteries)
Damage: upwards of 310
Armor pen: 20.
Move speed: +20% and more for Ghostblade
Lifesteal: 43%
CDR: 33%
Attack speed: around 2.0
Plus, an anti Mundo device known as ExCalling

In summary.... WIN.

How should you evaluate the Pros/Cons? What you should really do is take note of the pros, and understand your role. Also, take note of the cons, and understand what situations to avoid. Know your limitations!

Pros:
____
Fast initiation
Easy stun that deals great damage
Very great at catching up to a running champion
Very great at running away if needed
Ult can be used in defensive and offensive situations
Great farming Q + E moves

Cons:
____
This build really doesn't address his lack of health
Very weak, defensively
His E move can only be used once to escape, rather than twice to initiate (assuming you don't hit a creep on the way out)
Very weak to enemy CC
You WILL be focused

I usually leave runes up to the player. I wouldn't recommend changing your generic rune pages that much, as that can get annoying and expensive.

If you were to change up your runes, however, I would invest 1/3 in Health, 1/3 in Crit damage, 1/3 in Armor Pen. I usually leave 2 of the quints to be health with Renekton.

The mastery tree for this build is basically designed to fulfill a few different options.

Deadliness (3/3)
Burning Embers (1/1) [Upgrades ignite spell, +10 ap, use before ulting]
Sorcery (4/4) [Gives 3% cooldown]
Havoc (1/) [increases base damage, and magic damage by 5%]

The additional points put in defense were to slightly increase your armor, magic resist, and dodge chance. I did this assuming you do not have the runes to do so. If you do, invest in Veteran's Scars (4/4) [+60 health]

Here is the final build:
Infinity Edge
Phantom Dancer
Youmuu's Ghostblade
Bloodthirster
Executioner's Calling
Boots of Lucidity

Item order:
First, pick up a Vampiric Scepter and go to the laning phase.
After getting enough money, purchase Youmuu's Ghostblade.
Buy Boots of Lucidity.
Upgrade Vampiric Scepter to Executioner's Calling
Purchase a Zeal
Purchase Infinity Edge
Finish the Phantom Dancer
Top it off with a Bloodthirster

Easy mode.

Max out the W move first.
Max out the Q move second.
Max out the E move third.

Priority order:
R>W>Q>E

Viable options:

Flash
Ignite
Exhaust
Cleanse

Don't attack minions with more than half health unless you really want to PUSH a lane. Because Ren is so easily killed due to CC and Focus fires, ganking is a huge concern. Play it safe. Ward bushes. Ward the river. Don't push too hard with any MIAs.
Just use common sense, and remember Ren's Cons.

Getting last hits is really easy with this build. Just E in, attack, Q, attack, and E for a second hit.

If you are in a lane where you are constantly getting harassed and focused, do not fear! Ren is one of the only champions that cans till thrive while being so heavily harassed. Just wait til creeps get low, E in, quickly Q, and E back out. That simple.

When entering a battle you need to understand your role. As Ren, you should either be playing as a Carry or an Anti-Carry. This build is focused on playing as a Carry.

Many people, for some reason, are building Ren as a complete tank. They are building him a LOT like they build Nasus, in other words. While this MAY work in SOME cases... it won't work in ranked games. The job of a tank is to inflict so much cc as to annoy the enemy team and force them to focus you. If you try to tank Ren, you will end up being what we call a "Soak". You have a lot of health and defense, but nobody will target you.

As a carry, you should be focusing on the weaker of the enemies early-mid game, and you should hopefully get fed by late game. By late game, it is only a matter of 3-4 hits to take down any enemy champions.

Let's imagine a teamfight is happening. Your tank just initiated or started taking fire from the enemy team. You should QUICKLY pick a target. Pick the target with the strongest attack first. This is usually the case, unless there is a very weak character you can focus first. Then, E in to them. W stun them, and Q to slice them. This should easily take them down. If they are more beefed up than normal champions, you can easily chase them as they run with your second E move.

Let's imagine you are entering a lane to gank. This situation is very similar to the one above, but you should be focusing on timing rather than clustering your moves. Q in, basic attack for a second as the start to react, W stun them, basic attack for a second, Q them, and chase (with a second E move if needed). This effective combo takes timing and practice but is unavoidable and so hard to react to.

Now, let's imagine the opposite option. You are about to get ganked. You see two enemies coming up on your rear. You ignored the rules and pushed too hard with too many MIAs! Don't fret. You'll make it out. If they approach from behind you (pushing you towards their tower), you are actually in a better situation to escape. Run at them, and as you get in E range, use your E move being sure to hit at least one of them. Now, if one of them has any good CC (Ryze, Ken, etc) you will want to W stun them. If both have CC: stun the highest damage dealing one, if neither have CC: stun the one that may be able to kill you easily. After W stunning one of them, quickly E past them, towards your own tower. It will take some running, and possible a flash, but you should be able to survive. If you get low health during a situation like this, use your ult to pick up additional health that will let you escape.

Ren's ultimate is a huge concern for me, personally. I have a lot of trouble deciding if it should be better used offensively or defensively. This build would suggest the defensive option, but I still am going to roll with the offensive one. Both are very viable, though.

Offensive: at level 3 of your ult, it deals 100 dps to all surrounded enemies. Counting average MR thats about 72-75 dps. Whoa. That is a LOT. It really makes quite a difference in a bunched up teamfight. In the event of a teamfight, you'll usually want to pop it as early as possible (but not too early to waste it as they decide not to commit) so that it deals the most damage to the enemies.

Defensive: You'll want to use your ult to either a. scare off enemies or b. save your *** with bonus health. Using Ren's ult defensively is a great way to lure enemies to a trap. Get low health, they chase into your own territory, turn around, ult, E, W, Q, E, dead. You can also use his ult to escape (in a way) finishing ults that would otherwise kill you such as Caitlyn's and Karthus' ults. It is sometimes very worth saving your ult, especially if you have Bloodthirster stacks built up.

Please note that this build is a Work In Progress. I am working on adding pictures, better formatting, and alternate item builds for this.

Please consider that when playing.