I have mixed feelings about it really, I am aware of it, and use it in my dot files, but I think it's quite a gotcha - just recently actually I've been thinking to remove it.

It catches me out when something's ignored I don't expect, and it's not clear why in the working directory/repo, only for me to remember about the global one.

It catches others out (or catches me out by their doing) in collaboration when say I've not committed something, not even really been aware of the potential hazard, and that's been desired; but then someone else comes along and `git commit -a`s it.

But then where it is particularly useful is myriad tools that fall back on git ignore in lieu of (or in addition to) their own ignore files...