I have to say I too was a bit disappointed with the sound, but I guess I shouldn't expect too much with a $70k car.
I also agree that the S65 sounds similar to a big American V8, but that should not be too surprising, should it? Even so, I would say it still sounds better than the S55.
The best engine note I've ever heard is Honda's 3.5 liter V12 F1 engine (sample below); too bad no street car (not even those hyper/supercars) has come close to replicating it:
If I have to pick the best sounding stree-legal car right now, the Pagani Zonda 760RS would probably get my vote:
I suppose it's too much to ask BMW to make the new M3/M4 to sound like either one of those two...