4 months ago I picked up a Diesel ('92 300D 2.5turbo) for a work car, and I like it a lot. Took some getting used to, but it's a great car. First off, it certainly doesn't run like a gas car-acceleration is not as urgent. However, it goes pretty darn good, and once rolling over 25mph it has quite good acceleration, if not neck-snapping.
As to noise/vibration/harshness, if you are inside the car it is virtually indistinguishable from a gas car, other than making a different engine noise on acceleration. Going down the road at virtually any speed, I dare anyone to be able to tell the difference. A very well engineered car.
I get nearly 40MPG with my D, and that's driving it fairly aggressively (remember, I'm accustomed to a 500E which is a bit more spirited in its responses). It started with no problem during the coldest weather, and that was without being plugged in.
Good luck in your decision.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car