I engaged in some retail therapy yesterday, and took my new 24 mm for a spin. In short, I'm happy with my purchase.
Weight - This lens hardly weighs anything, and as a hiker/backcountry skier / traveler this is a big, big plus to me, and the main reason I bought this rather than a heavy Sigma lens.
Macro - Before purchasing the lens I wasn't too excited about the macro feature. Macros should be shot with 100 mm lenses, and wideangles produce bad bokeh and minimal blur, right? Well no - chuck all of that out the window. At short focusing distances, this lens produces thin dof and beautiful blurred backgrounds. Because the background covers such a wide angle, creating a nice background to a subject is way different than shooting with a telephoto lens. I had a lot of fun experimenting with the RF 24 mm's macro feature, and I liked the results way more than I thought I would. This is not just a gimmick - this adds another dimension to what would traditionally be viewed as a pure landscaping lens. I'm surprised because my old Tokina 10-17 zoomable fisheye (APSC) also has this feature but I never liked the results.
Landscapes - This lens does not disappoint. The center is quite sharp wide open and the corners are good. At f/2.8 the center is exceedingly sharp and the corners are very good. At normal landscaping fstops (5.6-11) the lens produces corner-to-corner sharpness. Flare is very well controlled. Rendering is excellent. I did not really put the IS through its paces so I can't really comment on its effectiveness.
Astro -
@Frodo prepared me for this, and my results are consistent with Frodos. At f/1.8 the sagittal astigmatism (wings) is really, really bad - bad enough to be obvious on web-sized prints. It's not just the extreme corners that are bad - the area affected takes up most of the image area. Stopping down to f/2.2 doesn't do much to alleviate the problem, but at f/2.8 things suddenly get better. I'd say it is the minimum fstop for acceptable results. I tried f/3.5 as well, and this tightens up the stars even more around the periphery. Only a bit of sagittal astigmatism remains in the extreme corners, but you have to go look for it.
In short, for astro you need to stop down. Stopped down it is a very good to excellent astro lens. This is the only real downside to this lens, and one that I was prepared to take. I have a tracking setup so I can manage slow shutterspeeds. With its macro capability, this lens is versatile a lot of fun.
FWIW my previous astro lens was Tokina 14-20/2 (APSC) which is sharp wide open and has very little coma/astigmatism, but this lens has field curvature which causes the corners to blur. On top of this is some decentering, which means that each corner can be made sharp but only by focusing them individually, which is an impossible workflow. I love this lens but it is very frustrating at the same time. The moral of the story is that there is always a tradeoff somewhere.
Attached astro photo: 100% crop of top left-hand corner. Extreme corners are top left. Shot on an RP at 1600 iso, 60 seconds tracked, some jiggle due to wind.