Sharp but with drawbacks

Just some background information, I am a landscape photographer and I have been using this lens since it came out with the 5DSR.

The major strength of this lens is that it is incredibly sharp corner to corner. I've also heard and read that this lens is a significant improvement over its predecessor.

My personal problem with this lens (which DEFINITELY doesn't apply to everyone) is its heavy vignetting wide open. During the day of course it's easily fixed by stopping down or in post. However, the problem becomes much more obvious at night. I enjoy shooting a lot of astrophotography and was excited to find out that this lens renders stars very well-sharp, no evidence of coma. But due to the combination of the vignetting and the camera's limited dynamic range at high ISOs, any attempt at correcting the vignette in post would render unusable corners. In my case with the 5DSR, the corners become very very purple. After some testing and research, I found that the corners of the lens produce near 4 stops of vignetting!

Another thing to take note of, I use this with a LEE filter system and the vignetting does get worse.

So in conclusion, it's not a jack-of all trades landscape lens. If you shoot mainly during the day at low ISOs, then by all means, you will be incredibly satisfied with this lens. However, if you are like me and likes to venture out at night, then I would suggest looking somewhere else. Perhaps the new Tamron 15-30mm, which I don't own but has read raving reviews about.