Interesting. I’ve owned my 453 for almost 4 years and 30k mikes. Is the engine as smooth as the straight 6 in our 1990 E Class Benz or the H6 in our Subaru Tribeca - no. However I don’t have a problem with its characteristics as an I3 either. Is there a very slight bit of vibration at idle? Yes but I find it totally acceptable and not excessive. At speed I find the engine reasonably quiet and it doesn’t feel stressed at 60, 70, 80, or even 90. Wasn’t so in the 451. I guess it all depends on what is acceptable to you.