It's all very well as long as you stick to exactly what someone else has already cooked up.

Try modifying the repositories or adding your own stuff and there is a world of hurt there. In the end, I found it easier to port applications out of the rPath repo onto RHEL than to get stuff working with Conary.

This sort of sausage-machine builder is already available for various architectures - that's what RH's Kickstart is all about., for example. rBuilder offers a neat-looking interface to such things, but the amount of effort behind the scenes mecessary to support this is excessive.