This could be pretty useful bc of the touch screen + pencil combo. Back in 2016 I spent a weekend mocking up an "iPad as a graphics tablet" app, and it was surprisingly easy and effective [0][1]. I'm sure Apple could do a lot better.
To be fair, I never use it for something requiring complex input, but it’s nice for something like a movie, a music app, or something that runs in the background but you might want to have a glance at every now and then.
It'd be nice if you could initiate it from the iPad side, though. There used to be a remote access feature built into iCloud called Back to my Mac, not sure why it was killed.
Also it's a bit finicky. For instance if you are using sidecar and the system goes to sleep, waking the laptop will not wake the ipad, and if things are asleep to long you get an unclear error message and have to restart sidecar from a system menu a couple levels deep.
It's an awesome feature, and using an iPad as a second monitor makes for a really awesome lightweight setup for mobile working, but I wish they would invest a bit more in the UX. It's already 80% there, and most of the fixes required don't seem super complicated.
I hope they make it evolve further, but it’s the story of Apple features. For the remaining 20% there are less seamless but more evolved third party apps (I personally use Duet, which is not without faults but has portrait support and more)
I use it with an older iPad so newer version might be better at it, but I see some artifacts on scrolling sometime, and it's clearly using the iPad's resources at full. There are params like resolution and processing that can be adjusted to priotitize what you care about (e.g. I go with retina res and more latency/lower processing).
I think sidecar will be better at the thing it does, I use duet mainly for the fexibility.