I mainly bought this lens for wide-field astrophotography and sunrises/sunsets. I find the imagery good at f/2.8 but noticeably improved at f/4. Stars are nice and sharp at the center, but hold up surprisingly well at the edges and corners, with only a small amount of blue flare visible. The Milky Way is absolutely beautiful through this lens, using a tracking mount rather than a fixed tripod. Set the ISO to 800, compose the shot, and take 5-10 exposures of 30 to 60 seconds each, depending on light pollution. Then use Registax to add them together to reduce frame-to-frame noise. Once stacked, I use Levels and gamma in Photoshop to reduce light pollution and bring out the best in the composite shot.
This lens is indispensable for those occasional sunrises or sunsets where the colors extend all over the sky. It's especially effective with a combination of sunsets and lakes or the ocean.
The only downside is the loose-fitting lens cap. I may try to bend it a little so it stays on better.
Overall, a really good lens for a really good price.