I'm really splitting hairs here, but I'd rather give more details than fewer. The TLDR overall is that it's an excellent user experience.
But, depending how well you can detect motion, it is not 100% fluid. The 16-inch has slight intermittent UI stutter when on the iGPU that you may or may not notice, but not because of a lack of performance and it seems to be intentional. Rather, this is presumably done for efficiency (powerful CPU + hard limits on battery capacity...well, you gotta make some sacrifices to get 10 hours of runtime.) It tends to exhibit this lag when the iGPU is completely idle, and then you immediately attempt to do something (e.g., move a cursor-->slight skip, press a key-->slight delay on when the text appears on the screen, attempt to scroll-->slight jump, etc.) When the iGPU is above its idle point (e.g., you have motion in the background, such as a video), this behavior does not exhibit itself. This behavior is also eliminated by engaging the dGPU. A workaround I've found to stop this microstutter is to use something like a desktop clock with a moving second hand, as the iGPU is then continually working. Like I said though, it is minor and many do not notice it, and while it annoyed me a bit at first, I've adapted to it and it has become less of an issue. I'm also usually on the powerful 5600M dGPU, where the issue is not present. High resolution playback on the iGPU is excellent--video is smooth, temperatures are low, and battery life is stellar. Overall, the UI is obviously going to be a bit smoother and more responsive on the powerful dGPU, but that's the reality with any dual graphics laptop given both macOS and Windows 10 both use some graphics-intensive UI animations.
The switching process is mostly seamless, although some users have had some wonky switching experiences where the screen blinks or there is a slight freeze. I had this issue with my first 16-inch with a 5500M dGPU on 10.15.7, and it seemed to disappear with 10.15.8. My second 16-inch with a 5600M dGPU did not have this at all. For the most part, it works extremely well. I really like how when I am using three displays and operating on the dGPU, I just pop off the displays, the 5600M switches automatically to the iGPU, and then I am good to go. No app crashes, no need to close and reopen...just pop off the displays and everything resumes on the iGPU. Likewise, applications needing the dGPU automatically trigger it, they work, and then when you close them the dGPU switches to the iGPU. In the long run, I assume Apple Silicon will eventually get us to the point where the need for dual graphics may cease, but until we reach that point the current implementation is a pretty solid solution.
