I know I know, he isn't called a "worthy pick" in ranked games .. but he just seems so fun to use (and why not, very unique in the way he can even break out of supression).
I'm trying to decide between him and Katarina. I currently own Annie, Sion, Sivir and Master Yi, so I was trying to finish my lineup with full game damage dealer (I only go jungle Yi).

So uhm, up to what point this guy can be used? I've seen nothing but negative feedback about him and his ult.

All he can do is KS with parrrly
Still a fun character.

uh ... gp can be a real problem for a ganker with his denies ... he like pushes the lane back and won't let anyone gank that lane.

If played right he can dominate early game and turn godlike end game.

There are people who took him to 1800+ elo

A Gangplank with runes is a force to be reckoned with, in my opinion.

That, and the fact that his deny is a godly tool in the laning phase.

Amazing laner if you have the runes. 30% critrate at level 1, crit 200 at level 3. See if anyone tries to harass you.
Denying is also stupidly broken, so that too.

I actually find gangplank underrated bigtime. Considering that he's is the only character that can TRULY deny in this game it's pretty nice. Parley gives some nice harrassing. You can pretty much gimp the opponents 2nd solo.

Farm top for 30 minutes, KS wiith Ult while farming, teamfight once, win or lose, GG

Thats how you play GP.

Fed lategame GP can be as scary as a fed Tryndamere. He won't have the extra 5-secs-before dieing-ulti, but if built correctly he can still crit pretty hard, get a couple of kills and heal/cleanse himself even out of Malzahar's ulti (yes, Remove Scurvy removes suppression, working as intended).

Also, the RNG ulti can *sometimes* help teamfights even if you are not there.

Fun hero. Hard to master though, you need to know how to give him a good early-mid game w/o crippling your team or yourself.