Finally with Ubuntu 12.04 we have a smooth Unity experience. I've used the 12.04 since beta and it just works.
They lost me to Mint in their last couple of releases.
I don't have any numbers for that, but even if it's a small shift, then it could be that most people weren't too fussed anyway.
That said, Unity was buggy in 11.04, and only somewhat less so in 11.10, and I still find myself having to occasionally escape to a terminal and restart Unity after it freezes up. I'm hoping 12.04 is better.