In a meta, where lethality dominates all (or so I'm told), I'm surprised that this build is not used by many. Why is Garen so good at the moment? Well, the reason for this is also the reason why you should be playing him at the moment rather than reading this fluffed out introduction. He does a lot of damage, like a tonne. In case you don't know why, there is a simple reason, his e (the thing that makes him spin).

A.D Garen is a Mid-Late game beast, you will have a bad time early unless the enemy has downs, just try not to give any kills, and let them push you in so you can get easy ganks.

PROS:
Great Mid-Late game
Does a **** ton of damage
Armor can't stop you
Passive is great for shrugging off damage in lane
CONS:
You are terrible early
Not much of an escape
Without core items, very weak

Once you get enough items, farming will be a sinch. In lane however, take what you can get, but play safe, let them push you in, you can farm pretty well under tower, especially once you have your B.F. sword. Once you do enough damage, and get your lane pushing, steal the jungle monsters, your jungler doesn't need'em

A.D Garen > Gromp > Your team > Enemy team
This is the order of damage in a late game teamfight, you are the top.

... it's Garen, he ain't too unique, he has a cool looking execute, a spinny move, an auto attack reset, and a move that reduces damage. Rito really stretched the creative department to create Garen