Here's my two cents, FWIW. I am not a fan of Unity or GNOME shell. (If you are, great. I'm not, and I'm not going to use either of them, full stop.)
I thought that, when Ubuntu 12.04 was released, I'd be switching to Xfce, or maybe Cinnamon, as my main desktop environment. Then, lo and behold, Ubuntu included GNOME "Classic Mode" (aka "fallback mode") in its repositories
; they fixed their "indicator applets" so they work with classic mode, and they sort of fixed the Ambiance and Radiance themes so they worked with classic mode (although Radiance looks better than Ambiance, and neither is without glitches). They also apparently did some work on Compiz in classic mode. So classic mode on Ubuntu 12.04 (with Compiz) is what I'm using now.
The things "classic mode" needs the most, IMHO, are the promise of long-term viability (although it sort of got that with its inclusion in an LTS release), and theme support. The underpinnings feel solid.
The problem with Mate is that I want a desktop environment that's fully compatible with the applets and applications that come with GNOME 3. Correct me if I'm wrong, but who's writing anything for GTK 2 any more?