Mastodon are too weird to describe... and they only have 1 full length album. Remission... you should definitely check it out though. Lifesblood is good as well... but just an EP.
and if you like Mastodon, you should listen to Lethargy. Bill Kelliher and Brann Dailor were in that band as well.
Wat genre do you think they should belong in if they do fit into any? I'm just wondering what type of music they play. I'm assuming metal, but mixed with what?