I play Garen as a roamer. Push out my lane with spin2win and then sprint to mid or go for the enemy jungler and put pressure everywhere.
Darius ulti also does more to targets that stack armor, since it's true damage based on the stacks of his passive. Targets that get a lot of hp (read cho'gath/singed) are better suited for Garen since his ulti scales off missing hp points. Building brutalizer on Garen is hilarious because no mana (also black cleaver rework in S3 but that's not for a while). Brutalizer also builds into youmuu's so that I can get to who I need to very quickly and keep them out of a fight. CDR, flat armor pen, and crit too, all very nice things for Garen.
On Garen, I tend to build boots 3x pots->HoG->boots of choice->avarice blade->sunfire cape for extra everything that Garen is a fan of->youmuu's/randuins. Youmuu's if I'm having trouble keeping up with people or getting into fights, otherwise I go for warmog's->finish youmuu's. Tends to work nicely but games never last that long anyways lol.
But hey, don't listen to me. I probably don't know what I'm talking about at all, I don't actually play this game a lot.