Hec offers a lot of great assets to the team, you can gank immediately after 3 camps, you have immense pressure from the early game and while you do fall off late, it's a lot less than some of the other early junglers like lee sin. Not only can you force many ganks with your e, you can easily repeat gank a lane and easily lane gank. Furthermore, he is one of the best teamfight junglers in the game with his w and ult. Mid game, with your e, you have a lot of mobility and can easily pick off that solo adc farming bot for free objectives later on.

His only downside is that he can be quite hard to get back into the game once you lose your lead.

Hec has REALLY good clears. Always start Red, raptor, Krugs then gank that lane, or red, krugs and raptors then either gank mid or go to your blue side. If you did raptors last and went to blue side, you should do wolves then try to take skuttle. If you see the enemy jg, judge if you win the 1v1, look at which lanes have priority (whoever is winning will come to help the 1v1 sooner)then decide if you want to fight or not.
I don't suggest doing gromp. Just farm and look for gank opportunities.
Before 14 minutes, your priorities should look like this Gank(get kill or flash)>Rift>Counterjg>farm>dragon>plates.
Dragon isn't good early on because it will take your entire hp bar and takes too long. Only do it if your laners insist and is helping you.
NEVER, and I mean NEVER take plates from your laner. Just go do a jungle camp, it's worth more, and your laner gets extra plating gold. Only do it for first tower.
Once you get Warrior, your prio looks like this Gank>Dragon>Rift>Counterjg>Jg

Wait for your team, ult their backline, smack their carries for a bit, once you go to around 90% hp, then press w. Stick onto their carries until they die, then work your way back.
Best scenario is to simple get a pick mid game with your e

When you gank, try to press e, run behind them, then autoattack, as this will push them into your team.
When you ult, people are feared for 1.5 seconds, and they'll run away from where you are during the 1.5 seconds, not where you were. Even if they are next to you, you can run behind them, and they'll switch directs and run towards your team.
Your Ult pauses the duration of your e. You can use your ult as a gap closer to successfully land your e.
You can press e to catch up to someone, ult behind them and fear them towards your team. During this time, you should not autoattack, but simply press q. After the fear wears off, you can then auto attack them with your e, which will push them further back into your team. The range of the knockback is about the distance of lee sin's kick.