I have a single $GOPATH per project and always recommend doing so. If you vendor code with a tool like godep[0], having one shared $GOPATH is a nightmare. Other tools might work better, but this is basically the standard for the projects I work on. I basically just have a zsh alias[1] that combines z[2] and sets the $GOPATH to $PWD split on "src". I do share a single $GOBIN set to $HOME/bin, though.
[0]: https://github.com/tools/godep
[1]: https://github.com/jzelinskie/dotfiles/blob/04ad026f30782189...
[2]: https://github.com/rupa/z