I moved to 2020 a few months ago after years on 2016 and a few months on 2018. Some thoughts:
- UI is slower in some places, but the command panel has been sped up for UI redraw and especially subobject switching, so better than 2017/2018/2019. Still not as snappy as old versions probably. There's an issue with a few UI elements since 2017 where you need to click on the dialog first so it registers clicks at all, annoying sometimes but one can get used to it.
- OSL is definitely a good thing to have. Not using it much as of now but planning to use it more
- Chamfer modifier has been improved - as the whole world knows by now :D - but it's really a huge timesaver and in 2020 it has seen some good improvements
- 2020 seems to be pretty stable, similar to 2016 or better, no crashes other than bugs from daily builds and such
- you'll love the viewport speed if you have used 2016 before, it's *much* better, this alone is a good reason to switch
- if you need fluids sometimes, they're in now
- customizing shortcuts has been made much easier with a new dialog but prepare for a surprise on your first launch, some defaults have changed
I wouldn't want to go back to 2016 anymore.