Also a fan of go-flags, I use it by default most of the time (unless the use case is so basic that the stdlib flags usage would be sufficiently simple).

I think my only real point of "frustration" (mild at best) is that flag composability isn't always possible (eg embedding/sharing common non-root flags across flag structs), IIRC struct tag eval doesn't reach nested types.

Other than that though, it's one of those nice libraries that just does a thing and gets out of your way.