On this GS 3.9.1 install, I don't see an ounce_of_difference between Gnome-Shell and Unity -- which is a good thing.
IMO, GS used to be a kludge. The only reason I kept it around was out of idle curiosity, and for a fallback DE, when Unity would break.
These days, I consider Gnome-Shell to be equal to Unity in every respect, including performance.
That said, I judge that ppl are really missing out, if they haven't tried the lastest Gnome-Shell. They've definitely made many, many improvements to it, over the past few months. And, there are more changes coming almost daily. Gnome is on a roll, right now.
If someone hasn't used Gnome-Shell since (say) 3.2, I think they will be shocked at the difference(s).