This is one hell of a rock album. Having heard earlier Mastodon material one would expect something a bit harder, but this is more like progressive rock mixed with metal elements. It sounds very much like old Black Sabbath at times and the vocals aren't too far from Ozzy either (back when he had a voice.) It is insanely complicated stuff, yet very accessible and highly recommended.
It's one of the best metal albums I've heard in a long while, even though it's not really metal, strictly speaking. Hats off to Mastodon for daring to create something like this. It'll probably piss off some fans, but if you have an open mind and are willing to listen then this is excellent stuff. My only reservation is that it could use just a little more aggressiveness at times to really push it to the limit.
The epic songs The Czar and The Last Baron are really outstanding and the latter runs for 13 minutes! But really, most of this album is outstanding.