I actually just recently got out of a game where I was Shyvana was was doing very well. Full build was Sated, Swifty boots, Bork, Trinity, Randuins, Bloodthirster. I was the villian, and I thought I would fight the Garen. For reference, this build 2 shots the ADC, and takes about 3 seconds to rip through their jungle gragas. Now you know why I was the villian.
Well, Garen and I are both full health and we begin fighting. I drop him to around 5% health when I fall to around 50% health. Then his ultimate hits. For the remainder of my health. 50% of my health gone, to true damage.
Garen is really strong right now. VERY strong right now. If I had to guess, I would say that some part of him is going to get nerfed before too long (mostly the killing tanky champions from 50% health part). So it becomes a question of whether you like Garen or not. He is certainly more than viable and is very strong at the moment.