DSLR Fisheye lens advice

Apologies if i come off a little amateurish. I'm still finding my way. Anyway, i've got a D7000 and would like to take some fisheye snaps. It's a bit of a gimmick, i know, but depending on circumstance, i think these pictures tend to look really cool (and i haven't seen any in the critique threads). I've been looking at fisheye options and noticed there seems to be two methods to achieve this.

  • (Some sort of) lens adapter that screws onto the end of an existing lens
  • Buy an entirely seperate lens

The lens adapter method is considerably cheaper - there are a whole bunch of sub $50 options on eBay etc - and i don't have high hopes for the quality there. I do see the advantages of being able to screw this onto any lens though and at an impulse-buy-price, i'm intrigued. I'm curious to know if anyone out there shoots fisheye shots with their dslr and which lens approach you've taken. If you've toyed with these cheap adapters or chosen a pricey lens can you share some of your experiences, please? I'm keen to hear some of your thoughts.