My Xbox one outputs a 1080p signal to my 720p 52" Samsung plasma as well. It looks incredible to me. My plasma has pretty nice color compared to my 1080p 22" PC monitor. Also the downscale process happening in my plasma really helps to alleviate jaggies in some titles (Forza 5/6 for example).
The one thing I don't like about my plasma is game mode completely takes away all picture options in my menu. I don't even bother using game mode because it looks so washed out. I don't notice any added latency.
I have a 50" 720p Samsung plasma myself. Yeah the only thing I dislike is the resolution, but this downscaling stuff for consoles and PCs really solves it. I love the response time of the plasma panel, and the color is excellent too.