I love Mastodon. I got into them when their first album came out so i manage to get some stuff that i could sell for quite a lot of money, especially in more years to come. Crack The Skye is great, every album has been a natural progression from one to the next, you have to hear it for yourselves from the Lifesblood EP right through to Crack The Skye. So in that sense i wouldnt say its a rip off anybody because they've always done their own thing, and gradually changed from each album to the next. Blood Mountain by far is the weakest album. Its good but its got nothing amazing there for me. Leviathan is definatly the album to own.
Best songs to look out for would be Trainwreck, Battle At Sea, Iron Tusk and anything from the new album.