Metal grunts (often humorously called cookie monster vocals) are good in small doses, or when utilized well in a band like Amon Amarth or Atheist. My personal favorite application of grunts is in this particular song: (starts at 3:26)
What I can't stand, however, are these lame, uninspired deathmetal/grindcore/goregrind/porngrind bands that do nothing but play as loud and atonally as possible while making the most indecipherable grunts, growls, and gurgles they can. Oftentimes, the lyrics are entirely pointless, as it's obvious the 'singer' isn't even bothering to try and vocalize any of them. That, and their subject matter gets old quick. Ok, I get it, you like to pretend to sing about blood and gore. It's not shocking anymore, it's BORING. Unless you're trying to be funny or legitimately scary, like a good horror or exploitation flick, you're not impressing anyone except your 13-year old, pimply-faced fans. This below is a prime example of the bland, monotonous shit I speak of. Don't listen to it unless you want to be bored to death by a wall of sound.
And yes, I know many of them are joke bands, like Anal ****. They may be a joke, but their joke's not funny.