I have that same 12x12 SSS on my TC40A. I just passed 50 hours on this tractor, and like the transmission just fine. My previous tractor was a '99 NH 1920, also with the 12x12. No problems with the transmission in it either. Shifts smooth, and sure is nice having enough gear choices to have just the right gear for what you're doing. My primary use for my tractor is bush hogging, with the occasional tillage, grading, and loader work.
In all fairness, I have to say I've always been partial to gear transmissions, so maybe I haven't given the hydros a fair shake. I have used several hydros of different brands, but just don't like them much.
My two cents worth, and worth everything you paid for it...