It's one of the most controversial features of the car actually.
I've always been in two minds about features like that.
A car like the M5 should, really, have some natural character - a bit of extra noise won't really harm its refinement to a degree that nobody will want to use it on longer distances. That's kinda the point of the performance ones really - a quick, refined car that has a naughty side. The preferred option for this is simply not putting as much sound-deadening material in, and giving it a noisier exhaust.
But this is where the M5 belies that it's just a 5-Series with some different bits bolted to it on the same production line as other 5-Series, rather than being put together in an M workshop somewhere. It'd be far more effort for BMW to do the preferred method than it would just to bodge a system using the car's speakers (another standard 5-Series part, natch).
And ultimately, the end result is kinda the same. I've driven the M135i with the sound symposer and at no point did I even think about the fact the sound was "fake". As far as I was concerned, it was a bloody rapid little car that sounded great but wouldn't be hard to live with.
...At which point you come back to the reality that the M135i, or the M5, is designed to be used every day, and the symposer is a nice way of doing that. The same could be said of a GT86 or a Fiesta ST which do a similar thing, albeit with a rather more low-tech bit of plastic that transfers induction noise into the cabin.
So the fake sounds don't really affect the cool rating of the M5 for me. The fact it's a 5-Series is far more of an issue. 5-Series of any sort only start getting cool when they've got a bit of patina and their first few banker/sales-rep owners have got bored of them and they start filtering into the hands of enthusiasts.
