To me this looks very similar to running a source based distro (Gentoo,Exherbo,...) with lots of packages using git as a fetch method (Gentoo uses the -9999 version suffix, Exherbo calls them scm packages).

Can anybody tell me when I should use gittup instead? Is it just a matter of size? Is rebuilding significantly faster with gittup due to the automatic rebuilding?

I mean to me it looks that gittup has some serious limitations in its current state (e.g. no 'make') while the others are full blown linux systems.