I've been working on a new design for this site. As a part of it, I'm experimenting with SVG images in both HTML IMG tags and in CSS backgrounds. I really like the way that they scale as a part of a responsive design.
I haven't yet had time to compare suitable PNGs with SVGs to determine any cost or benefit in terms of file size. I would guess that the SVGs are likely to do better with simpler designs and fewer colors. Perhaps I will substantiate these guesses someday.