I jumped from 17 to 19, and was in 19 only briefly before moving on to 20. It felt like the biggest jump by far was from 19 to 20. My perception is that 20 is pretty stable now -- not perfect, but stable, and I REALLY like 20 better than any past release. It's taken me some time to feel fully comfortable with the new interface. I think it's worth the upgrade for the added Properties and Favorites features, which you can appreciate very quickly. However, in the long-run, Graphic Overrides probably are going to be the biggie going forward for you, and it will take some time to wrap your head around this concept, which I sometimes still struggle with a bit.
If you have a good window to upgrade now, my advice would be to go for it so that you can spread the learning curve over two releases, which will be far less painful. 19 to 20 was pretty painful as it was, and I wouldn't want to add the learning curve of 21's features on top of that.
Richard
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Richard Morrison, Architect-Interior Designer
AC26 (since AC6.0), Win10