I wasn't sure what to make of Mastodon when I first heard them. Unbelievably heavy, but my first impression was that they were a band that had a great sound; now all they needed was some great songs. I no longer have that view, as I've found a few songs that I could listen to over and over, but I must still admit the tempos are complicated and the riffs are ten bars long, so a single listen might leave you thinking, WTF?
Give it a couple though, and you might think these guys are masters of musical time and space.
Mastodon - Mother Puncher