I've used 9, 10, 11, 12, X3 & most recently, X4.
Strange as it is, all of them are as buggy as hell, except for 11 (which I use all the time)
As each newer version comes out, they come out with some pretty awesome features, but damn dude, you try use those effects and such and bam, CD crashes and so often, I've lost some hectic work.
So... Till i see a CD that doesn't crash alot, i'm sticking with 11.