The  other popular package manager I haven't seen mentioned is MacPorts.

    Oh yeah, I've used ports from time to time too. Thanks for the reminder!

    I think only one of these, brew or ports, uses /opt as a default.
    The other uses /usr/local.. I think?

    Sniffing around, might it actually be that (mac)ports uses /opt,
    and (home)brew uses /usr/local/opt?

    The times I use package managers are when I'm installing tools that have complex
    dependencies on other projects, and magically pulls in the right versions and builds
    it all with the least pain, avoiding having to carefully pick through the package specific
    README/INSTALL docs on how to build for each platform. There's so much variance,
    and so often build errors incompatible with the local compiler.. it's great that brew/ports
    handles all that.

    I don't find myself installing packages from source with wide dependencies often.
    Most of the libs I build against are minimal dependency-wise, like FLTK.

    This is on purpose, so my apps don't get forced to specific hardware/OS versions;
    the more dependencies, the more restrictions.. esp. with things like movie libraries
    that sometimes depend on specific compiler features, hardware for playback, or
    take advantage of particular features only available in newer OS versions.

    And any apps that have many dependencies, like image/movie viewers, photo
    manipulation tools like GIMP, etc. I often install as prepackaged bins; it's usually
    utter madness to build those from source, due to the massive dependencies.