I do wonder how much the aesthetic will change.

I can really talk about the aesthetic changes due to using the shell-theme-upstream "theme", which now gives the option to have or not the rounded corners. It looks weird at first, specially after years of being used to having one, but we'll get used to it. It's not a make-or-break thing for the desktop.

I also wonder what those corners will look like when a window is maximized

It always filled the corners whenever a window was maximized, but the rounded corners of the top bar were drawn over everything the desktop shows, like a GIMP layer that sits on top of every other one. Now that it is gone it just shows a the regular corners of the window, like this:

Image