One of the things I tried really hard to do is make the HTML _really_ clean for things like this.
Is reader mode "really good" or do you think it loads very nicely because of the work we put in to make the HTML nice?
I think it's a mix of both. Honestly I vastly prefer Reader mode's presentation than any other layout, and especially any layout which changes if I interact with it (resize window, move mouse, click mouse button, press key, send window to background, whatever).
I have seen some sites that completely break when using Reader mode. I have seen sites that are very well done in Reader mode complete even with pictures.