Great lens, covers FF, OK for astrophotography at f/2.8 and good at f/4, heroic vignetting, the mustache distortion as noted earlier, and it will have a less-expensive filter solution soon. I posted about the filter set-up a few months ago here.
Most importantly, you get more smiles per buck from this lens than any other lens. Bargain, even if you are just using it occasionally.