It is awesome. Incredible power. Silky smooth on highway. Sounds like a beast on start up. I regularly get 36/37 mpg on a long drive.
I prefer the sound of the B58 in the m340 over the S58 in the m3 but they are quite different engines. And way different power. But straight-six throughout.
Still think one of the best engine sounds out there is the s54 in the 2001-2006 m3
At the moment, I have a Boxster, and the flat 6 somehow feels smoother than any I6 I've owned. I've done some research and sure enough, the inline 6 is (mathematically) smoother than an inline 6. The only way I can explain the fact that the boxer feels smoother is because it's mounted low and behind me, away from steering components? Whereas in my I6 cars, with the engine in front, you feel it through the front subframe and steering a bit.
Smoother in what sense? My Boxster is stick so the shifting experience is very different. The flat-6 is a remarkable sound - intoxicating. Especially purely naturally aspirated though I don’t hate the sound of the turbo 3.x variations in various Porsches. BMW ZF is great but it’s not the same.
Otherwise, yes - the handling in Boxster is so different compared to the m340. It just feels like it’s on rails around corners. No body roll. And for me peak way up in RPM means my highway merges are an absolute blast / symphony.