So,

I got Maokai the second he came out.
It took me awhile but this is the build I finally came to use with great success.
It's designed to allow for early harassment and then move into some good mid to late game damage.
The item / runes / masteries aren't meant for tanking. They're there more to give you some good survivability for when you get in close with twisted advance.
I like to think of this build as a beefy caster kind of thing.

For runes i got straight magic penetration and ability power.
The magic pen combined with ability power makes your saplings extremely powerful for early game harass.

Went 11/19/0 for the masteries.
Only went down far enough the offense tree to grab the magic pen.
The rest of the points were dumped into defense for survivability.
Alternatively, you could skip the defense for the utility tree for more... well utility.

Start with the ring to enhance that early game harass via your saplings along with the bonus health and mana regen.
I then move straight into sorc's shoes to increase movement speed (Maokai is sloooow) and for the added magic pen.
After I have the shoes I move right into the Rod. It bonus ability power, mana, and health make it a great all around item for Mao.
Next, I grab a Soul Shroud and Morello's Evil Tome for the ability cooldown. In addition to the ability cool down, both items provide great mana regen along with health from the Shroud and a good boost of AP from Morello's.
Finally, I grab a void staff to add to the magic pen and ability power I've already acquired.
Matches usually end around this time, however; once the aboce items have been purchased, I'd recommend selling the ring in place of a Rylai's.

I usually end the game with around 15 kills, 5 deaths, and 10 or so assists.
If you use this build it's very important to remember to keep the pressure on the team early game with sapling harass and ganks. Also remember this build is not meant for tanking, it's an AP/Survivability Build.

Please add any comments or feedback you see fit.