PROS:

-Probably the best ganker in the game
-Duskbringer and Unspeakable horror can be used offensively and defensively
-Great farmer
-Dominates lanes
-One of the best junglers as well
-Can carry your team
-He's just incredibly awesome

CONS:

-Easily thwarted by CC and targetting
-Somewhat squishy until Frozen Mallet

Nothing I hate more than to join champion selection to see someone already take Nocturne, and then get in game and they have some ******ed crit based build. Nocturne is exactly like Caitlyn, physical bonus calls for straight AD. With this build you can lane against squishies and dominate the lane with relative ease.

I take armor pen Marks and Quints because having that armor pen at the start gaurentees first blood and dominating your lane against any champion who's squishy. Even tanks or beefy champs who aren't ready to take the damage you're about to output.

The Glyphs and Seals are because they generate about 9.4 attack speed when taking shroud of darkness first. This starts you off with great attack speed and can be thrown over 1.0 if you get the shield right. Synergizing well with your passive, you'll eat away at the seconds before it refreshes to deal another devastating blow. One passive generally takes about 1/4 of most champs hp, assuming they didn't get the hp crystal or something of that sort. Another bonus is once you get ghostblade you will be at 1.6 atk speed and can be boosted more from shroud and activating ghostblade, so you only need one atk speed item.

Many Nocturnes get a point in Duskbringer first, which is nice for the damage and movement buff, but doesn't really compare with the damage your passive deals. You can do two passives in the time it takes to cast another duskbringer, while also dealing your 73 atk damage per hit in between (Thanks to armor pen runes.) Any squishy dps in the start will die in seconds. I've had countless triple and double kills just at the first blood gank on 3v3.

Ghost is probably the most useful thing any Nocturne can have. Often times when you ult, people run. Ghost allows you to keep them in your fear.

Exhaust. Honestly, I use this more to keep champs from fleeing than actually using to overcome 1v1 fights.

I've done this a few times in situations where a squishy is about half hp and they have a carry next to them: With the skill set of Nocturne paired with his summoner spells you can easily jump on the squishy with paranoia, set unspeakable horror on the carry or tank, ghost after the squishy if you need to and if you're still in chase exhaust the carry/tank until you get the kill and then turn around and waste the other one.

Done this many times, in 5v5 and 3v3. Most memorable was ulting a nidalee at half hp, fearing jax, ghosting and duskbringer for the kill, exhaust jax and start demolishing him. You can also string combos like this in 2v1 situations where you are ganked. Keep one off while you kill one, then turn and take the other. You're lifesteal and combo of spells should keep you alive long enough to get both kills.

Once you have a few points in duskbringer and a few AD items: Lead with duskbringer down a row of minions, run to the middle and autoattack a minion to activate your passive. Late game you can push lanes in a matter of seconds.

Generally I play 3v3. I lane bottom usually so I have quick access to the lizard and so that as soon as I wriggles there will be someone defending top and I can solo the dragon.

-AS SOON AS you get madreds, get the lizard (and top jungle buffs if lanes are pushed)
-AS SOON AS you get wriggles, solo the dragon while others defend the lanes (finish jungle buffs if you haven't)

Doing this puts you ahead in gold, gives you buff advantages, and sets up ganks for paranoia. (just remember to ping lol)