I drive a mapped 335d and yes it hurls you down the road at an astonishing rate but it does so with so little drama that you have no emotional appreciation of the speed.
I daydream about the throttle response and noise of a naturally aspirated straight six manual.
As far as running cost go, I get 28mpg around town and 40+ on longer motorway journeys. Not too much better than a 330i I suspect. The 335ds do have lots of niggles with their twin turbo set up, comparable probably to the petrols fuel injection / ignition issues, but expensive issues are fairly rare.