Rengar shouldn't have trouble with garen, his slow completely negates Garen's Q. I played against a good rengar and it's impossible to get near rengar. It turns into a farm lane with a kill edge to rengar.
If you lead with Q against Rengar of course he's gonna beat you.
Rengar is really a feast or famine type of character. There's this red who plays him a lot (or used to think his name is xelnath) and said that when he did well he stomped hard but when it didn't go so well in lane it really sucked. He's a decent champ, but other assassins can do much the same with better escapes (and building tanky isn't that great from what I've seen)