What Black Metal does to religious sensibilities, Grindcore (wikipedia, allmusic) aims to do to auditory sensibilities (i.e., essentially rape them).  This stuff is intentionally difficult to listen to, both in a sonic sense and content-wise.  While some groups have a genuine social bone to pick (no doubt related to the politics/social commentary inherent in the Hardcore Punk influences betrayed by the subgenre's name), others are out to shock and offend as much as possible (-ahem- Anal Cunt).  This extreme iconoclasm is of course highly attractive to many Metal fans, and so the offensiveness of this music should not automatically be interpreted as a criticism.  I actually don't hate listening to Godflesh (which is more than I can say for other samples of Grindcore), and I have to confess that Anal Cunt's album and song titles are hilarious (I mean c'mon..."Recycling is Gay"?  "I Like it When You Die"?  "I Gave NAMBLA Pictures of Your Kid"?  Priceless!).  Still, I probably won't be adding any Grindcore tracks to my next dinner party playlist.