Samyang 7.5mm vs DMW-GFC1 vs Cheap £40 fisheye converter from ebay?

I'm currently on the lookout for a fisheye lens for my GF3, I have the 14mm pancake so I was just thinking of getting the DMW-GFC1 when it comes out. But I never realised how cheap the Samyang lens is (only £80 more than the converter), but i'm wondering what the main differences are going to be? Also, why not a cheap converter from ebay vs the panasonic one?

Thanks for any help anyone can give!