I've owned the f/2.8 version of this lens for many years. When I began taking astro photos last summer, it was one of the first pieces of equipment I grabbed.
Canon uses premium glass and coatings in their L series lenses and its a fantastic lens just due to the flexibility it gives you.
I'd say there is a slight amount of chromatic aberration when compared to my prime focus equipment, but the flexibility of going from 70-200mm is worth it IMO.
I already had the f/2.8, but if you're looking to get something new, purely for AP, I think the F4 is perfect. I won't often use mine wide open anyway. Also, if I decide to pair it with a cooled astro camera, I'll be stuck using the larger aperture.