I find the 3.0 MV6 a very fast car even with an autobox, they are big heavy cars and the bullet proof smooth GM autoboxs suit it well.
Funnily enough I don't find even the manual all that quick, 210 BHP isn't anything special in such a big car, and the gearing is too high for an engine with peak power at 6,000 RPM. What they really need is a 6-speed box to allow the ratios to be lowered without comprimising cruising; a 4-speed slushbox and torque converter is exactly the opposite!
The GM autobox is far from bulletproof, it may be good by normal slushbox standards but a quick search of this site will reveal that failures requiring complete box replacement are not uncommon. I don't consider the R28 manual box to be a particularly strong unit but it's still loads more reliable than the autos from what I can tell.
I know there have been a few fails, but considering the amount out there and the amount of cars they are in. Thats not many.
The AR35 in my Senator is still silky smooth @ 141k miles, and no noise from it.
Without maintenance any box will fail, but the boxes by GM have been quoted as the finest in the world.
The rate at which my Senator goes from 30 up to 3 figures is incredible.... why you need anything more powerful than that for regular road use is beyond me! :-/