It is the exact opposite imho. You definitely hear the engine and exhaust more outside, but since the cabin was designed to be very quiet you can't hear the engine/exhaust at all so BMW had to bring the noise to the passengers. Almost every car manufacturer is doing that nowadays - Peugeot, Ford, Dr. Ing. h.c. F. Porsche AG (911, Cayenne), Lexus and so on. Sometimes it is a simple channel going from the engine to the cabin (case of older Pugs) or more complicated things (see Pork). You can turn down the symposer by putting the car into comfort mode.
As long as it is only a system that enhances the real noise I don't mind it. But, and that's a big but(t), BMW has clearly gone more cheaper and artificial way. The problem is, as we know, that they recorded the sound of the engine, tweaked it a bit obviously and put it into the car's software. So what you're hearing is about 40 % real deal and the rest is fake-ish real deal. Do you want a BMW that plays a fake noise? No. Does the M5 sound bad inside the cabin? No.
I wonder why BMW didn't want to design a system like Porsche's for their new cars even though they had this sort of thing in the old Z4 3.0si several years ago. It wouldn't be that ridiculous and would do the job equally good or better. Maybe money was really the object. By the way, not only the M5/M6, M135i, M550d do have the ASD - in late 2011 they started putting it into the Z4 roadsters, then F12/F13 640d and will probably continue in this in the future. According to realoem.com other cars seem to not have it, not even facelifted 5 Series, 7 Series, 3 GT etc. The new X5 is still not listed on realoem, though.
As we know, you can plug it off but it also disables the whole sound system including radio, bluetooth and whatnot (does not apply to Bang & Olufsen in the M5 and M6)... HOWEVER, some German M5 owner has found out that there are probably options in the car software where you can change the reproduced sound so it can play some diesel N57 noise!
In my opinion, the whole ASD thingy is totally stupid. Specially in M cars.