Using a little Opteka fish attachment, I've gotten some very cool shots. The problem is that all shots get, of course, progressively less sharp as you move out from the center. I know that this is to be expected, especially with fisheye lenses and wide apertures. However, the blur is so bad that any photo taken with the attachment is completely unsuitable for printing.
So, here's the question: If I go buy a Canon/Sigma dedicated fisheye lens, how much better will the quality be? Worth the (steepish) price?