Just got it today! It's kind of unexpected, anyway, it's a UW model. I will try to compare it with my Canon 200mm f2.8L with a Kenko 2x very soon.
This zoom should be far more flexible, and at the same time, it has IS which is great for daily purpose. For astronomy, the Canon 200mm f2.8L should be better since it could be used wide open. The Kenko 2x is more useful for solar eclipse trip before, and now I guess maybe the Canon 100-400mm f4.5-5.6L can replace it entirely. But of course, I shall try it out first.
So if you're interested in the comparison between this two options at 400mm f5.6, stay tuned.