Akali is most definitely one of my favorite champions at this point. Who doesn't love a little ninja girl who can fly around kicking people's butts? Honestly though, I picked up the idea of Akali in my first game, the mechanics aren't overly tough. What i failed to realize my first few times (Yes, I was a bit of a noobie, but we've all been there) was that Akali is so much more than a mexican jumping bean who destroys squishies. Personally I've seen entire games turned around by an Akali.

I go with the Greater seals of Vitality simply because I feel ever so slightly safer with a bit more health. It provides me with that extra bit of health to knock out one of there players and dip out before I die.

The Greater Marks of Strength are for your passive. I generally don't go for the flat Ability Power/ Attack Damage simply because I feel like it limits me ever so slightly. I'll admit it's tempting to have those nice stats early game, but i prefer to build more for my late game and I don't feel hindered at all not having the flat runes. With your very nice Doran's Blade, you have your life steal passive anyhow.

Greater Marks of Alactricity, simply for that slightly faster swing after your Mark of the Assassin. Debatable whether or not you should get the Magic Pen. instead, but this is just my preference.

My masteries are based around getting my passives early, as well as a bit more survivability (only a bit) with the extra armor and magic resist. I've seen a mastery build further into Defense, with 10 points into offensive, but it just doesn't fit my play style personally.

Doran's Blade : Wondering why an AP champ with a Doran's Blade? Well ask no longer. With the additional 10 AD, Akali gets her passive spell vamp. right off the bat. Neat right?

Boots : simply for that extra bit of Movement Speed to get you places :)

Hextech Revolver : I find the extra bit of spell vamp. keeps me in my lane longer farming (or in those tricky fights) on top of somee extra damage. Akali is all about staying ahead, and that means less time going back and more time getting many minion kills (or slaying your enemies)

Rylai's Crystal Sceptre : All the health you need, packed in with Ability Power, and of course, not to leave out that slow which is utterly essential for picking off players.

Sorcerer's Boots : just for a bit more damage :)

Rabadon's Deathcap : pretty simple here, all the ability power to instantly destroy someone. plus all that extra spell vamp. from the extra damage

Sheen : You may be asking, why am i getting a sheen? it has mana? Well it adds a bit of Ability Power, and you hit for 100% between ability spamming! What's not to like?

Hextech Gunblade : debatable, some players would say it's not worth it's cost. I however do like the gunblade, the 25% extra spell vamp., the ability power, the extra AD to add to your passive spell vamp. and a ranged slow/damager use.

Lichbane : more of that beautiful Ability Power, extra mobility, AND magic resist, on top of the sheen activate? rounds off your build quite beautifully I'd say.

I generally go for exhaust and ignite, although flash is a fine alternative in exhaust's place.

Ignite: I put a point into my ignite in my tree, so when used it gives a bonus 10 Ability Power while it's on cooldown, allowing me to have both my passives up early game.

Exhaust: Makes early game ganking / killing considerably easier in my opinion.

Flash: A ninja who can tp into a tower, destroy you, and flash out to safety before you blink? what's not to love.

Pros :
Akali has a lot of damage, and amazing spell vamp.
Hard to focus (in most occasions)
great for chasing down those champs who just love to get away

Cons :
Your early game can be shut down, if you can't get any kills, or zoned out of farm.
Squishy! must be a bit careful. only a bit.
good chance of being focused in team fights (mejai's increases this risk)

Despite the fact that you have huge damage, keep in mind you have a team. If you see you are ganking with a teammate and you're quite ahead in kills, don't hesitate to get the enemy champion low enough for your team mate to late hit it. There's nothing wrong with spreading out the kills, and actually you will see it helps in the long run. Can Akali destroy an entire team while they smack away at your allies? The answer is yes, is some instances. What happens when they realize "Akali carrying their whole team focus her?" well sir, that would be when you lose. Spread out your kills! Don't get greedy!

Thank you much for reading this guide! Any comments or suggestions to improve on this, will be very appreciated! Try to keep it constructive, this is my first guide! :)