As is the case with Doom Metal, the subgenre name for Drone Metal (wikipedia) is an excellent indication of the type of sound you can expect.  This stuff is very experimental and cerebral (in the musicological sense), featuring longish compositions based on musical tones (it actually reminds me a bit of the atonality of 20th-Century classical music).  All of these samples are exclusively instrumental, which I think is fairly typical of the subgenre.