I’ve used it a bit. I like the week view on the iPad and the day view on the iPhone. I’ve ended up shifting away from it because the app seems slow to launch. I often want to pop open my calendar for a quick peek, so perhaps more than other apps, any time spent staring at a splash screen is a big negative.
I couldn’t deal with the slow transitions between views - it wouldn’t slide, rather would fade in/out and reveal a new view. I like having scrollable views that help with continuity.
I think it’s pretty good, especially in comparison to some, ehm, overhyped competition. What I like about is overall a good overview - for some reason I don’t feel overwhelmed with a mess which is my calendar (work, several private ones) even on an iPhone in the week and month view. Speed is not an issue on my iPad 2018 and iPhone 8+.