dunno what build others use, but I really rarely have a problem with mine. boots of swiftness, ghostblade, phage and then it'll depend on the enemy team.. I can go frozen mallet / atmas, hexdrinker or most of the time i'll end up with a last whisper. If noone builds armor (which rarely is the case) i'll go straight for infinity's edge.
Garen is highly underestimated.
He is highly mobile, durable, disruptive with his silence and can do some really decent dmg to mix it all up. specially with the new item coming that will give him even more CDR. 38%cdr Garen with masteries, watch out.
I guess most Garen rush frozen mallet and that's what makes them ignorable.