Faster Mac OS X 10.10 Yosemite Upgrades for Developers
Your Yosemite upgrade may take many hours if you’ve got anything non-Apple in your /usr folder (Homebrew, Texlive, or Mactex for example).
I don't think the solves the problem I had with the Yosemite install, but this article has good suggestions for anyone using Homebrew, etc. Note that this also applied to Mavericks so this might be useful advice again in a year.