My 335i has been butter smooth from day one.
Even when stopped in "D"/sport AT, there is NO vibration anywhere, steering, seat, or chassis. It's so smooth you can't tell if the engine is running or not.
I have ASS turned OFF by default.
When I have used ASS, shut down and restart were also quite smooth, much smoother than all the 328i's I test drove.
The N20 is also a smooth engine, but there is a bit of vibe that comes through, but only at idle.
Those of you with N55's that have a "rough" idle, does it smooth out once the engine is warm? With MT at idle, does the roughness go away if you put the lever in neutral?
This engine should be extremely smooth at idle, if not I'dd take it to the dealer.
BTW, when you do feel the roughness, what idle speed do you have?
Does it fluctuate a bit up or down or both? It shouldn't.
"Hanging" rpm can happen if there is an intake leak somewhere, and that can also contribute to a rough idle, and fluctuating idle.
Have a dealer tech look at it, and fix it.