Now contrary to popular belief DO NOT get Mejai's first (This is actually only good for a karthus lol) Start out getting that Delicious Hextech Revolver, it's got spell vampirism, which will increase the damage and the heal from EVERY PROC of your life drain. Grab some boots, I used to build them into swiftness, but the rune page extra 1.5% helps. Plus level 2 Ionian boots with cooldown are just too amazing to pass up.
The void staff also increases the damage that fiddlesticks does, because it allows for EVEN MORE health steal and damage. Plus Fiddlesticks AP ratio's aren't exactly anywhere close to good. Now, getting The crystal scepter is a good way to beef him a little, and the slow again procs off of lifedrain.

Naturally, ever since Rabbadon's Deathcap came out (The RapeCap) any caster has been balls deep in AP, so it's always a good thing to get!

Fiddlesticks is a glass cannon. While ending the game at 18 with the low health score of around 1800 is a problem, The League's retribution to this, is that fiddlesticks can do over 1000 damage a second, if built right.

The Skills I have selected for him make him a great ganker. Getting life drain first is crucial and it will allow you to stay in lane a lot longer. Unfortunately, fiddle is not a great farmer without Dark Wind, but this is my drop skill for him, and you want to get it as late as possible. The classic 1-2, fear-life drain combo is still in effect for this build.

To play fiddle, one must play in the grass! After you get level 6, make sure you have enough mana for the Crow storm, fear, life drain combo. (about 450 mana at level 6) Stand in the grass, when the enemy champion gets close let them push past you a bit, or if you have flash that will work as well (you can select flash while crowstorm is channeling, so if they are too far away you can crowstorm and then flash right up onto them). Crowstorm in, immediately fearing them so they cant get away, followed by popping life drain for an additional tick on their life source. Now if its a squishy champion (I.e. Misfortune, Lux, etc.) then this will bring them down with ease. Void staff should bridge the gap for all the others as you increase your AP.

Remember though, one more second of fear is way better than another 50 damage with life drain after you pop your ult, which is why you build a level of drain per 2 levels of fear. Here's a tip! CROWSTORM JUMPS OVER WALLS. if them enemy champion has pushed your mid to the tower, just get behind that turret wall and crowstorm in, hitting them with fear and life drain. This is the perfect recipe for an instant win!

At Level 18, fiddle can farm minion waves with ease using his Dark Wind, but again while the 1 second silence would sound good using it for ganking or early combat, it just isn't good enough compared to his other skills. But, when it can bounce up to 5 times, that one second silence can turn the tide of a team fight pretty quick.
In team fights, wait until it's been initiated before crow storming in, this will lessen the chance that you will be targeted in the mayhem, allowing for survival and maximum damage. The worst feeling ever is Ult-ing in and immediately being focused(Zhoyna's will help a lot, but usually it's not really necessary). Always remember to be careful, Fear and Flash are great ways to escape danger in any game, the same goes here. a 3 second stun will get you away, as well as flashing over a wall or two.
I've updated this guide March 23, 2011. I'm still going completely positive with this build, sitting at a last game of 15/3/2. Hope you enjoy Fiddle!