I have produced [Jigdo files](http://znark.com/fedora/jigdo/) files for [Fedora Core 6](http://fedoraproject.org/wiki/FC6ReleaseSummary). [Jigdo](http://www.atterer.net/jigdo/) is a program which can produce ISO images from the constituent files and a description. It uses a .template file which contains the description how to compose the image from the files and the missing pieces, and a .jigdo file which describes where to find the constituent files.
It allows downloading the packages from the mirrors which can be faster and easier than getting the images and producing the images from them. It is also more useful to keep the package repository and then produce the images as needed. I know some people keep the images and then mount them to get the files.
One thing I noticed is that the the disc1 and DVD templates are much larger than they should be. The cause is the repodata directory in the images is different than the one on the mirrors. My guess is that the output of createrepo is not deterministic; the contents seem to have different ordering but the input packages are the same.