This album is regarded as a seminal stoner rock album (along with Sleep's Holy Mountain (Persona 101: I disagree that Holy Mountain is a rock album, but whatever)) and it's easy to see why with the instrumentation/production. The guitar is a wall of chunky heavy sound, the bass rumbling underneath and a slight hollow sound to the drums creating a lazy attack on each beat.
Now a thick, heavy sound is all very well and good, but what of the songs? Well, tracks such as 'Green Machine', 'Apothecaries' Weight' and 'Writhe' should answer that question for you as most tracks on this album are very heavy and, perhaps more importantly, catchy .
Overall this is an album that is always heavy (but not metal) and mostly catchy/interesting. A criticism of this album is that it's a bit too long and by the end I find myself tired of the album (as the sound on this is album is pretty much constant).