GTK3 apps won't play nice with the global menu, I haven't run into issues simply because I'm on an old distro (a Ubuntu 11.04 [Natty] based distro) which is all GTK2 and official support has actually EOL'd for this particular version.
I manage because security wise i'm covered and Apps wise the only thing I'm missing out on are apps like Audience etc (new GTK3 app). My machine isn't exactly new (It's 4+ years old...maybe more ) so I'm not too concerned about the additional features the new kernels have brought. The _only_ real issue I faced was getting updates to Firefox beyond v17, and that I fixed by moving to the Nightly builds which are still compiled for Natty - I must tell you, they're AWESOME - so much faster and actually very stable despite being cutting edge than the release versions. And then, there's always Chrome/Opera which continue to be supported.
Lastly, I have eOS Luna beta up and running on another HD on this system for my GTK3 needs
I'm in the middle of an "interface crisis here", and since I use Arch, I have always the latest software (and most of then, GTK3).
I LOVE XFCE, but Gnome 2 seems much more complete to me. I miss the little default system configuration tools (like the integrated network sharing for instance). Once upon a time I used to like GNOME3, but now... Blargh.
Well, maybe I'll give MATE a shot. Thanks man, keep your good work.