Mayhem/YMMV

Epic Riff - A lot of the time, most notably on De Mysteriis Dom Sathanas.
Evil Is Cool - Arguably part of the reason some fans like them.
Face of the Band - Dead and Euronymous, due to their untimely demises giving them a sort of evil mystique.
Love It or Hate It - Amongst fans of the black metal genre as a whole, everything Mayhem recorded since De Mysteriis Dom Sathanas is like this. ...Sathanas is almost universally regarded as the pinnacle of the genre (or at least, of the genre at that point in time), but the polarizing reactions to their more recent work is absolute, and best illustrated by Ordo Ad Chao (a solid attempt to get back to their darker, lo-fi roots, or a particularly lazy rehearsal tape with full production) and most of all, the aptly named Grand Declaration of War (genuinely avant-garde prog-black metal which showed a willingness experiment with unusual time signatures, guitar techniques and electronica or Jesus Christ that chap spent the whole album just talking!).
Magnum Opus - Fans consider De Mysteriis Dom Sathanas to be this; it's often regarded as the greatest black metal album of all time.
They Changed It, Now It Sucks - Grand Declaration of War and Chimera received this reaction from many of Mayhem's fans.