1999.12.01 16:17 "RE: libtiff 3.5.3 release.", by Peter Smith
In my experience, CCITT G3 and G4 perform well only for certain kinds of images. I can't define for you exactly what will and will not work well (maybe somebody else knows this?), but I do know that images with a lot of small, randomly placed features (like noise) will not compress well at all.