I just played him first time ever and went 17-4-22. Team went 45-38-63 as a whole.
It was the most thoughtless game I've ever played. I went 2-1-0 in lane and then bullied Riven out as often as I could and ganked mid relentlessly when I forced her back(seriously, that's Garen's purpose. Ganking the AP Mid. Jesus the silence.)
My mid took his tower at 10 and was able to roam and put pressure on bottom. Me and the jungler took dragon every time it was up. When they try to gank garen, if they piece it together poorly it just results in their jungler dying. If they do it somewhat well, you can just sprint-balls-to-the-wall out of it.
That said, it's kind of fun to play thoughtlessly on occasion. Garen might be my new troll character.