Pros
Rammus is extremely potent against "Auto attack stackers" such as Trynd, Yi, ect.
He is great solo.
If you use smite he can be an effective jungle ganker.
Cons
He is VERY squishy against Ability Power users when not in Curl(Buy MagicResist to help this)
Until you get his Ult, he's not Super effective.

Runes are simple. Magic resist to fill the void of Rammus's low base Magic resist, and Attack Damage for more damage output.

I choose Exhaust and Ignite together. The damage reduction is good when you are outnumbered (which is hard to admit on Rammus). Ignite for "The ones who got away"

Grab smite if you're looking to jungle, it can speed things up early game.

Ok first off you have Ninja Tabi. I pick these up as soon as possible for the dodge and armor. Next I usually go with ThornMail, though this is a situational item. If you are faced with a predominately Ability Power team, build something like Force of Nature first for the Magic reduction. Now we come to SunfireCape, this is great for when you gank. Roll in with Powerball, taunt, sit back, and watch the show. The last thing I'll touch on is Rabadon's Deathcap. This is totally up to you, I like it for the extra damage reflect and damage onto his Ult. Though don't be obliged to get it, feel free to pick up another Armor item for the extra Attack Damage bonus from his passive.

Power ball first to get those pokes early game. Then start swaping between Taunt and Defensive Ball Curl.(Focusing on the one you feel you need more that game.) Getting his Ult at 6, 11, and 16.

Everyone plays champions a bit differently, this is my take on Rammus.
Early game play it safe. Initiate with powerball then back out, essentially "poking" at the enemy. Once you reach level 6(aka get your ult), you can start ganking. Be aware of your map and identify "gankable" targets. Run through the jungle to reach you targets unseen, once near you target start up powerball a bit early, the movement speed ramp up will get you going pretty fast by the time you get to them. Hit them with powerball, then immediately puncturing taunt them. After you taunt them pop defensive ball curl and your ult. This all should happen within 4 seconds of hitting them with powerball. The result is one dead champ.

I'm also going to talk about Rammus's effectiveness against AutoAttack carries such as Master Yi, and Tryndamere. When facing these champs build Thornmail RIGHT after making boots. Players using these champs will take you as they would any other champ at first and go for the 1v1. Let them hit you a few times, once they start to feel the bite of the Thornmail they're going to want to backoff. That's when you intiate the Taunt, DefensiveBallCurl, Ult sequence. With the mass amount of damage reflection going onto them they usually will kill themselves in a matter of seconds. Using this method I've easily 2v1ed Trynd and Yi simultaneously no problem.

That wraps up my tactics for Rammus, and this Guide. Hope you enjoyed!
(Comment for things you want me to Add)