Grunkies on their own are mildly more interesting than giant rats, because players haven't encountered them before, and in their overlord form are dangerous due to the multiple attacks. But really, there's nothing particularly exciting about them on their own - they don't have anything really cool going on. Very few of the monsters I've put together for the first-level party do, of course - all that cool stuff will kill them just the same as a 1d6 bite attack, so all the best abilities get saved for higher level monsters.
That doesn't mean there isn't a way to spice all this up a bit more.
The beggars who infest the City Underfoot have seen a hideous shape swimming beneath the filthy waters - "Near as big as a man, yet it was a grunkie! Tore poor Gorm's throat out, and nearly swiped me pine-liquor!" Not having a background in post-apocalyptic zoopathology, these beggars are entirely ignorant of the horrible affliction that is Grunkie Overlordism. They tell tales of men turning into grunkies under the light of the full moon (how this light penetrates into the sewers is left unasked and unexplained), and how only silver can slay these man-beasts.
Indeed, on two occasions the crowds of beggars have caught this were-grunkie, and slain it with their pitiful daggers, incurring great loss of life and moonshine. They toss the corpse triumphantly into the sewage, but each time the beast has soon returned, and its predations grow more and more frequent.
Anyhow, that's all I've got right now. The eBay sale has completed, and now I'm in shipping hell. I'll put yet more junk for auction up next week, but I need some time to recuperate from this batch first.