I'm fairly sure it's actually called 'Apple Mail'.
Yes, I use it and Yes, I dislike it.
I started using it when I got an iPhone, to take advantage of the integration between phone and computer. That part works well.
It's graphic design, typefaces used, layout is uglier than the original Gmail, which was visually well done.
I soured on Gmail when Gmail changed from their original simple interface to a gimmicky, commercially oriented one in which Google sorted your mail into their categories.
Also, I didn't want to be logged in to Google any more than necessary.
The stacking behavior on Apple Mail is somewhat adjustable, but overall the layout, graphic look, and behavior is at least somewhat annoying.