farbfeld is a pretty nice format indeed. I've been involved a bit in the project (I ported the JPG driver from the "imagefile" format to "farbfeld"), and I must say it is a really nice format to play with programatically.

Something should be told about this format though, as we're bringing up the tests. farbfeld always uses 16 bits per channel (RGBA), while PNG might not. For JPG, it only uses 8 bits per channel as far as I know. As it also lower the quality of the image, it is better at creating "patterns" that would allow a better compression. In order to compare FF to JPG, I think you should take a high-res picture (in PNG for example), and convert it to both FF.BZ2 and "lossless" JPG. This way, you'll see which one is the best.

Other than that, good topic! I've personnaly been using FF for quite some time now (got a script to convert all my pictures to FF before viewing it) and I must say I never really wait before seeing an image. Most of the time, it takes longer to download the image than converting it (I use lel as my image viewer).
I also take all my screenshot in FF format with xscreenshot. Now all we need, is wider adoption!