I'm using the 12.9 as a tablet - without the keyboard. I love the big screen. It's great for brainstorming (using the Apple Pencil) and as a second display. I haven't actually seen a lot of movies on it (only one) but the experience is nice. As I come from a 10.5 iPad Air 3, I think I've made the right choice. It takes some getting used to for working on a 12.9 tablet but once you get over the "this is too big" period, you end up loving it.
One important thing to mention: the 12.9 has a considerably smaller footprint than the 13.3 Air. Of course, it's in the name (12.9 vs 13.3) but the 13.3 barely fits in a messenger bag. The 12.9 fits comfortably in a case. However, the same case makes it a bit taller. It seems a small thing but it does make a difference. I have to struggle a bit to pull out my laptop, while I don't have to do this with the tablet.
One important thing to mention: the 12.9 has a considerably smaller footprint than the 13.3 Air. Of course, it's in the name (12.9 vs 13.3) but the 13.3 barely fits in a messenger bag. The 12.9 fits comfortably in a case. However, the same case makes it a bit taller. It seems a small thing but it does make a difference. I have to struggle a bit to pull out my laptop, while I don't have to do this with the tablet.