I must say, they're rather snazzy, aren't they? Been listening to them for a while, but I've just today gotten into them something hardcore. But this leads on to the real point of the topic...
I was having a debate with a friend of mine; do Isis count as post-rock?
He thinks they're 'sludge metal', but to me, they give off a very post-rock... feel. It's all sprawling and epic, and mostly instrumental. And even when the vocals do kick in, they're not really like any other metal-vocals I've heard. We kind of came to a compromise and called them 'post-metal,' but I'm still not sure.