Until the recent 4.18 update AP Gankplank was just a troll pick, but with the recent change to his ult making it more reliable than ever I can definitely say he's quite insane. Massive AOE damage for team fights and a monster laner. Running Sheen to proc with his Q in lane shutting down the enemy mid lane as well as heavy sustain with his W, making him nearly impossible to gank. I'd say the new GP is definitely viable.

Pros:
Good laner
Hard to counter
Great harass
High Sustain
Insane Team Fighting Ability

Cons:
Requires decent mechanics
Needs team to work with you
Without ULT not much damage put out
Damage over time (Only Parley does damage/unless you use basic attacks with Nashors)

Early/Laning Phase:
Try to farm up and harass every now and then. Use your Q to farm as well if you can't harass freely, Parley gives free gold when you kill a minion with it so it can help put you ahead in gold. Keep in mind you don't wanna waste to much mana so try to maintain 50%. Utilize your W perfectly, do not waste it unless you run out of pots and you're gonna die. Reason is if the enemy mid/jungle tries to cc it's your greatest escape without blowing summoners and usually 100% get away card.

Mid Game:
By now you should have at least your lichbane and beating the **** out of the enemy's mid. Remember your ult is Global, keep your eyes open on the map to help out your team mates and pick up a few easy free kills with it. Every once in awhile you may have to use it to clear a wave when you go buy but it's fine. The CD on your ult is not too bad.

End Game:
By now if you're not up by like 7 kills you did something wrong. Your parley should be hitting 550+ damage with your lichbane proc and your ult should be dishing out 2K plus damage on max output. This is the time were you get your team together group the hell up and force the enemy to fight in your ult. Remember to kite the enemy out with your Q. Now when to go in and out of the fight. SURVIVE! You're a sustain damage champ not an all in burster. Let your ult deal the damage and peel for you.

*Congrats you just won the game and the enemy is trashing their mid going "Are you serious? You lost to an AP GP!*