I'm 5'11" and I've got two 18" GT's and a 16" GT. The 18" frames are definitely a better fit for me. The 16" Karakoram is a little bit too short, which makes for a comfortable more upright riding position, but gets a little cramped over longer distances.
I also ride a 14" DMR, a 17" Marin, a 17.5" Orange and two 19" Saracens.