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

The Panasonic conversion lens is the Panasonic Lumix DMW-GFC1, it costs around £140 in the UK. http://www.panasonic.co.uk/html/en_GB/Products/Digital+Camera/Lumix+G+Lenses/DMW-GFC1/Specification/8534950/index.html?view=&angle=1

The £40 one referred to is almost certainly a third party item, which is almost certainly inferior to the Panasonic.

I agree, the Samyang is the only option of the three to give a diagonal 180° field of view and (from the images I've seen) has much greater optical quality than the two conversion lenses.
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I'm getting there...