Actually, it's very positive. And I don't recommend that you stop it down. I use it wide open. The lens is very usable wide open at f/2. It is not perfect wide open, but few, if any, lenses are.
I said, if you want to make the trade-off between exposure time versus optical performance, then you can stop it down, if having the best stars are more important to you than signal-to-noise ratio at a given exposure time.
And if you want to shoot things like meteors, or fixed-tripod starscapes or time-lapses, then there is no substitute for aperture speed, and I don't know of another 16mm f/2 lens that performs better than the Rokinon at f/2.
It just depends on what your priorities are.
Like almost all lenses, it gets better when stopped down. That's not exactly news.
It's certainly better than the Canon EF 16-35mm f/2.8 L USM AF Zoom at f/2.8 and the Canon EF-S 18-55mm IS AF Zoom at f/3.5.
And it's a stop faster than the 16-35, but it costs less than 1/5 as much.
The links to the reviews of those two lenses were broken in the Rokinon review (trying to convert everything over to HTML5 has been a nightmare).
Here is the link to the 16-35mm review, check out the stars in the corners:
http://www.astropix.com/HTML/I_ASTRO...ESTS/16-35.HTM
And here is the link to the 18-55mm review:
http://www.astropix.com/HTML/I_ASTRO...ESTS/18-55.HTM
Jerry