I absolutely hated gangplanks ult, until i realized it targeted anywhere in the world. Since then its been useful casting it on lanes where they least expect it, help with ganks, defend other lanes when they cant be reached, etc. Nobody wants to even risk getting hit by it because it CAN be devastating, so it is an excellent disrupter and forces people to scatter. You can use it to help encourage enemies to walk into ganks to escape the ult. In a straight up fight its unpredictable and frustrating, but the utility makes up for it.

Huh. I didn't realize you could cast anywhere, nor did I even consider combining it with Nunu/Amumu. I guess it does have its uses.

It still is very iffy regarding if it actually HITS the people you're dropping it on, but King Cool touched upon probably it's most prominent effect in that it can cause chaos and scatter the opposing team. Watch for team fights taking place on the map whether you're a part of them yet or not, and drop that ult in at the perfect moment.

well. karthus now... rip lichking