I did exactly what OP is considering... 2008 335i -> 2013 M5. I love it. It's a bigger car for sure, but it handles better. Certainly very fast. I'm quite happy with how it sounds with the windows down. Windows up it's harder to hear the exhaust, but I don't want to annoy the neighbors with a stupid loud exhaust. Performance wise, obviously it's way faster. I find the iDrive is current enough, but clearly not as good as the most recent 7-series / F90 stuff. Being in Arizona, having really really powerful AC is great.
I have only had the coolant lines leak (even though the recall was done) so the dealer re-did the repair. Other than that, I haven't had any issues at all. Burns very little oil, less than a quart in a year with ~5k miles.
Only problem is for some reason rear tires don't seem to last very long
