He's so hidden OP.
You literally just stack straight armor out of Cinderhulk (and maybe 1 Spirit Visage) and no one can kill you from mid game to late game. Ignore the Lethality hype I've yet to see a single game I played Rammus, made it to mid game, and was anything but close to immortal.
He tanks. He has insane mobility. And best of all, a super reliable point and click Taunt (POINT AND CLICK DUDE) that is perfect for targeted lock down and peel.
All you have to do is play the early game super safe (with the knowledge that in late game, you are infinitely more useful than Kha, Rengar, Lee etc). If they counter Jungle you, get help or play it safe and run, avoid fights early. Gank but make sure you know where the other JG is and never be in a position where you can get Counter Ganked.
Come mid to late game, Rammus will take over the game (unless your team is too behind or your team somehow picked a team comp with no AOE damage).

Enjoy the hyper mobility point and click OP front line.
You can also split and group really fast with his hyper mobility (ex take towers with his Ultimate and then group etc).