I've had the pleasure of running one round a track for the day, and have to say I was mightily impressed.
But I haven't bought one.
Yet.....
As far as looks go, I'll stick with the 916 shape, but, if I had the cash, I'd buy a 999S tomorrow, as the ride's exceptional, and the motor's awesome. But I can't justify buying another and keeping mine, especially with my other little project underway..