The thing with using turntables is that it makes it even more difficult to get the bike on the centre stand, and if you are one of the owners who struggle anyway (and lots do) then that just adds to your woes.
I have never used one, but with the amount of weight on my front wheel (even with my Sport Cases fitted) I don't know how I would turn it round.
Even with all my considerable (but less than it used to be) weight on the SW-Motech Alu Rack to the extent that my feet are off the floor, I can only just get the front wheel to lift. I certainly couldn't rotate the bike on a turntable under my bike.
Brian (who would love to see someone using it on a K12/13 without a bunch of mates or Guy the gorilla in attendance)