This guide is very out of date. Sorry! I would like to update it soon as my title is quite epic, just haven't had the time yet! Check back at a later time :)

Intro:

Here's the deal guys. I really loved playing Fiddlesticks before the patch. I was alright with him, hard to know for sure, but I think I was probably about 50/50 with him in the wins/loss column, but the recent buffs to Fiddlesticks make him an absolute powerhouse!

Pros:

1. An absolutely awesome jungler.
2. Phenomenal in team fights (surprise, triple kill!)
3. Excellent crowd control abilities.
4. Self-healing.
5. Most powerful damage-over-time damage dealer.

Cons:

1. Very squishy, easily killed if focused.
2. Poor Mana Regen.
3. Cast time on Ultimate Ability requires setup time.
4. Crowd control interrupts a lot of damage on his Drain ability.
5. No burst damage.

Game-Play:

Early Game Preference: Jungle

Doran's Ring is the first item of choice, and as a Jungling Fiddle with this rune setup, you will need the 10 AP that it gives. Another option is to take an Amplifying Tomb for late game upgrading. I prefer the Ring for extra survivability and Mana Regen.

Fiddle should kill the Golem first, but again, with this rune build, a little bit of finesse will aid you greatly. Watch this video to see. A few auto-attacks before the first Drain will give you the edge to finish with full HP and plenty of mana to jump right into to killing the Wolf-Pack.
Play Video
This video outlines my jungling method if I don't do any ganking before level 6 (Yi was just too low hp, I couldn't resist, even if he is a Bot hehe). Sorry about the video quality, I had a browser running, and I realized later that my virus scanner was updating, so the quality is poor. I added some fun/relaxing music to compensate! :D Enjoy.
Mid-Game: Jungle Clearing, Ganking, Lane Pushing, Over-all Floater

Mid-Game for fiddle, is really game dependent:

1. If my team is over-all winning, I will often continue to dominate the jungle while ganking as viable.

2. If my team is struggling in a lane, I will help out with controlling and pushing that lane to get a tower destroy, continuing to clear jungle mobs is still a priority unless the opposing team is just completely dominating.

Late Game: Team Fight Master

Fiddle is all about timing. When to join the fight is imperative. The key is to watch for ultimates to go off for the other team, or people focusing on other targets. Basically, you want to make sure that when you Ult in... you're not going to get auto-focused, controlled, and killed.

Items:


Doran's Ring
10 Ability Power, 120 Health, 5 Mana Per 5 Sec. Early Game Item of choice. Adds survivability, Mana-Regen, and Ability Power.

Sorcerer's Shoes
20 Magic Penetration. UNIQUE Passive: Enhanced Movement 2 Boots of choice. Magic Penetration is absolutely huge for Fiddlesticks' damage dealing viability.

Rod of Ages
80 Ability Power, 650 Health, 725 Mana. Passive: Your Champion gains 18 Health, 20 Mana, and 2 Ability Power every 1 min. Bonuses cap at +270 Health, +300 Mana and +30 Ability Power. Lots of added ability power, and survivability. In addition, the extra mana will make a huge difference since this build lacks a lot of mana regeneration.

Zhonya's Ring
120 Ability Power. UNIQUE Passive: Increases Ability Power by 25%. UNIQUE Active: Places your champion into Stasis for 2 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 sec cooldown. This item is incredibly powerful in conjunction with his ultimate. If you ult in and get focused immediately, you can pop this item to continue doing the damage that you need while staying alive.

Void Staff
70 Ability Power. UNIQUE Passive: +40% Magic Penetration. The +40% Magic Penetration is Huge! In addition to the 70 extra ability power, this is a must-have item for Fiddlesticks.

Abyssal Scepter
70 Ability Power, 57 Magic Resist. UNIQUE Aura: Reduces the Magic Resist of nearby enemy champions by 20. This item is excellent, it will add some extra survivability with magic resistance, and will assist you and your other AP damage dealers with nice boost in damage. Skip this item of course, if someone else on your team has it.

Rylai's Crystal Scepter
80 Ability Power, 500 Health. Passive: Dealing spell damage slows the target's movement speed by 35% for 2 seconds (15% for multi-target spells). I almost never make it this far, however if I do, this item is my Doran's Ring replacement of choice. It ups my survivability, and it ups the power of the ultimate by giving it a 15% slow. It also gives the Drain ability a 35% slow. A very, very nice upgrade from Doran's Ring.

Archangel's Staff
45 Ability Power, 400 Mana, 25 Mana Per 5 Sec. Passive: 3% of your Max Mana is converted to Ability Power. UNIQUE Passive: Each time your Champion uses an Ability their Maximum Mana will increase by 4 Mana. This effect has a 3 second cooldown. Bonus caps at 1000 mana. I will pick up this item if for some reason I don't grab Abyssal Scepter, or depending on how the game is going, I might get this item over Rylai's Crystal Scepter (but not often).

Summoner Skills:

Flash
Teleports your champion to target nearby location. This Summoner Spell is extremely important. It can get you in and out of trouble if you screw up on your ultimate ability teleport.
Ghost
Your champion ignores unit collision and moves 32% faster for 14 seconds. This ability is huge for keeping people in range of your ultimate, or getting away from a bad situation.

Masteries:

I occasionally go down the Defensive Tree for fun, but 99% of the time, I specialize as the guide suggests. I will discuss the masteries that I think might be unexpected to many. The other one's I'll leave as I believe that they are self-explanatory.

Awareness
Increases the amount of experience your champion gains by 5% (4th rank). I pick this mastery, because as a jungler your natural experience gain, puts you behind everyone else. However, with this 5% gain you should easily keep pace with everyone else, perhaps leveling faster than the side lanes.

Greed
Generates 1 gold every 10 seconds. This mastery is optional, but I always have a hard time arguing against extra gold. :) Let's be honest, at 30 minutes, it's only 180 extra gold. But as I said, I have a hard time arguing against extra gold :).

Utility Mastery
Increase the duration of Neutral Monster Buff by 30%. With all the jungling you should be doing, this mastery can prove invaluable.

Quickness
Increase your base movement speed by 3%. This mastery proves very valuable since you're not grabbing boots of swiftness, and fiddlesticks has an overall slow movement speed.

Runes:


Greater Mark of Insight x9
8.55 Magic Penetration These runes are imperative for any AP Damage Champion.

Greater Seal of Vitality x9
9.72 Health per level (174.96 at 18) Best Primary Seal for Fiddlesticks in terms of survivability.

Greater Glyph of Insight x9
5.13 Magic Penetration I used to use Ability Power per Level runes here. But I found that late game viability demanded that I switch to Magic Penetration Glyphs.

Greater Quintessence of Insight x9
5.67 Magic Penetration Again, magic penetration is so important for Fiddlesticks.