It generally regarded as equal or better than the Nikon. People are selling their Nikon's to get the Bower.
"The IQ is as good as with my best other fisheyes and tops them all at f/3.5 especially near the edges of the image circle, thanks to the specific "very low" radial compression in this area coupled with "sharp" vignetting. Take note that the image circle diameter is exactly the same as from a shaved Nikkor 10.5 mm. The Samyang has however 2 mm and 20° of focal length and FOV respectively less than the Nikon lens!" From: http://michel.thoby.free.fr/SAMYANG/Early%20test%20report.html
And what would be the advantage of shooting wide open with a fish? You don't get shallow DOF. The Bower is adequately sharp stopped down. There is no reason to EVER shoot wide open with a fish and expect the same image quality as you'd get stopped down.