Several reasons for buying an m5 or similar i guess:

You drive autobhans at 150mph+ a lot
You want to drive it hard
You want a status symbol (thats you phil, in your "phat gangsta ride")

For anything else, a fully loaded 5 series diesel is a better choice.

I dont do the first, i couldnt care less about status symbols, so only reason i would buy one is to use it the way JJ does