Don't like the size - you'll mess with the incredibly fine tuned suspension settings of the M5. Don't like the wheel design either. I'm going to try putting my BBS RS-GT's on my car, but they're exactly the same size as the OEM wheels - just forged and a bit lighter.
i like the m6 wheels better than the m5 wheels, but as many guys/gals as you ask, that's as many opinions you are going to get...but personally i think even a 19 inch wheel is pushing it as far as cosmetics go on the e60 or e63.
that ps work makes it look like a 21 incher in front and an 18 incher in the back!!