I have to agree with that epic rant that umiman posted. I've mostly avoided the playerguides and whatnot and just learned to build what works for me. I play MF as a pusher, building E and W almost exclusively. Q is the last thing to get tacked on, because it doesn't seem all that great to me, whereas I live and die by my E and R. This means my item build needs to straddle AP and AD, with a touch of cooldown reduction and lifesteal to boot.
So i wind up getting boots first, then building a Gunblade, then a Manamune. Then maybe a Frozen Mallet. Hell, past the boots and Gunblade I just play the items by ear based on what I'm needing.
Is it tweaked for maximum efficiency? No.
Does it work? Well, I now average 0-2 deaths per match (unless we're getting stomped hard), typically outlevel the rest of my team, and I can hold any lane solo for the entire first half of the match. And I usually have a huge lead in minion kills at the end of the match, and a significant amount of gold. Once Make It Rain gets amped up good, it's instadeath to an oncoming creep wave, with all that gold that entails. Which enables me to steamroll the item builds, which lets me kill all the things, which gets me moar gold, which...you get the idea. It's a beautiful thing once it gets rolling.
It's not a pure AP build, more of a hybrid build. It works for me. Even if I don't rock the Casbah and get 20 kills a match, I tie up people in my lane to the point where they're having to call in reinforcements and make things easier for my team elsewhere. Or I'm in ur base, shooting ur tower.
And then when the teamfights come, I can lay down a field of fire that does major damage, slows the enemy and usually sends them running for cover. If they group up and rush me, I toss my head back, laugh maniacally and unleash a cone of bullet death. Either way, we're that much closer to a win.