150% spellvamp will easily get you a large number of kills mid and late game and great surviveability.
You may ask, how is 150% spellvamp even possible?
Well, a part of Akali's passive, discipline of might, gives her spellvamp based on how much attack damage you have.
I tried this myself just today and i got legendary in a matter of minutes, in double and triple kills.
This is also a good build for players who tend to rush a lot; i am one of them, and i find that i can heal my self for over half of my max. health every time i use Crescent Slash in a wave of minions. You can also solo basically any champion (or sometimes two of even three) as long as you keep attacking. This also makes a great jungle Akali. I soloed dragon (on twisted treeline) at level 8 without losing more than 100 health in the end.

Akali is a great nuke, initiator, carry, and burst damage dealer.
She has a very offensive as well as defensive set of abilities (depending on how you use them).
For example, Twilight Shroud can be used to escape deadly ganks, and also to confuse your opponent in a duel. Her spells are best cast in a Q-R-W-E-R-R combo, which will devastate almost any opponent.

I build Akali as a hybrid (anyone would), but focus more on attack damage on my items, letting my runes take care of a part of my AP needs.
I also build more AD because of her passive, giving me immense amounts of spellvamp.

I take Mark of the Assassin first because it deals the most damage at rank one, and it also sides with AP, which makes instant use of my first item, an Amplifying Tome. It can also get you first blood really easily: you first hurl your Q, then wait for your team to get the target low on health, then flash right in and activate your mark. (yes, it's a little bit of a kill steal, but that's what we have to do sometimes)

Crescent Slash is the next most important to rank up. It sides with AD, so your gunblades and BF sword items should soon make this spell OP.

Twilight Shroud is the least important (but still very useful) to rank up quickly because it does not prolong the total duration, just the slowing effect it has on enemies (which isn't much). It is a great escaping tool, too. Run off into the jungle, cast your W near a wall, go into it, then just flash right over the wall and the enemy would still think that you were inside your W.

And of course, rank up your ultimate whenever possible.

Even though Akali has great escaping abilities, i still recommend flash, because as useful as her ultimate can be for escaping, it still requires a target. The many walls in the jungle are your friend. Flash through them while in your W, and nobody would suspect you haven't stayed.
In fact, the only champion i don't take flash with is Kassadin.

Ignite is always my favorite.
Great for last hitting when your ult is down.
Damage; no other explanation available.

I don't think there needs to be an explanation for this mastery build; almost every Akali guide promotes this.

I choose more of AP runes so that i can concentrate on building AD for my innate in-game so i don't have to worry about not enough AP.