I'm happy owner of Samyang 7.5mm/f3.5 fisheye for m4/3. The image quality is excellent corner to corner. However, the distant scale and the infinity stop are not very reliable. Thanks to the one-touch magnification of the LCD on GF5, the manual focusing is a breeze. The focus ring feels well damped but may be a bit easy to worn out.
I'm looking at 14/2.8 for my D7000 because the distortion is strong but of a simple barrel type on DX.
One complaint about the fisheye and 14/2.8 is that they have 6-blade aperture. So, when you stop down, you will only get six-ray star image. I would have liked to have seven-blade aperture.