Excellent little lens

As fisheye is something you don't use often this is excellent very affordable alternative for expensive Panasonic fisheye.
Even lack of autofocus isn't problem most of the time because in landscape/sky photography huge depth of field allows you to forget it after focusing once.

Optically Samyang is fully on par with Panasonic according to Lenstip which I can well believe.
Metal construction is very well done and solid with nice focus ring which is tiny bit stiff (actually helps to keep focus in place when handling lens) but doesn't have plasticky stiction. In overall beats lot more expensive m4/3 lenses like Lumix G 7-14mm.
And design being native for sensor and mirrorless keeps size and weight small so it's easy to carry in camera bag.

Besides Rokinon Walimex seems to be another brand for this lens in some European countries.

Problems:

No communication with body.
While you can't miss recognizing images taken with fisheye there's aperture data neither in EXIF or shown by camera if you're interested on checking exposure related parameters.