Technologically, it’s the best DE out there, no contest. (Maybe with the exception of touchscreen integration)
But some design decisions grind my gears so hard I can’t use it.
I get irrationally angry when I see the bouncing cursor animation, or look at a list of my programs and half the names start with “K”.
It feels too sluggish, overloaded and Windows-y in its default configuration and getting rid of everything that nags me takes too long, when Gnome comes out of the box looking simple and stylish.