I was offput by it so much on my last phone that I rooted it (first time rooting any Android) just to override it as much as I possibly could. For me, it wasn’t because of the flatness, but entirely because of the huge padding.

The volume slider, which was a thin stripe before, now looks like a comically large bar for no reason at all. Small circular icons on the notification shade which could fit 4 in one row, now only fit 2 in a row. Pulling down the notification shade still let you see the screen behind it, but now it grays it out entirely.

As for the custom color selection, the main gimmick of Material You, it is entirely hit or miss. On my own phone, Pixel 3, I used a red/maroon color, and on my new phone, Pixel 5, I use a mint green.

There’s situations where my chosen color looks really good, and others where it looks horrendous. As it turns out, having one universal color choice for things ranging from the notification shade icons, time display on the lock screen, calculator, etc. makes it difficult to find one color that looks good for all of them.

TL;DR: I hated it when it came out. I have gotten used to it now, but still dislike it immensely.