I recently switched to regular for my 95 E320. Here's why:
The old arguments why we need to stick with premium don't apply anymore for cars with knock sensors.
I poked around further to find out why premium is so much more expensive. There's no refining or other technical reason; the gasoline manufacturers simply believe the premium buying crowd are driving upscale vehicles, and they don't mind paying extra for their premium fuel. They're gouging the supposedly well-to-do folk.