I believe someone said it should be changed every 50 thousand miles, or 80 thousand kilometers.
As for the difference, vibration is down a noticeable amt (80% or so). Also, I have not yet really had the chance to drive it all that much to see how it does over potholes and the like, but I did go out tonight and it did seem smoother over bumps and such, not that much, but still smoother.
The old damper was worn and I could pull it and push it very easily, whereas the new one I had to put some effort into pushing and pulling it. It probably has never been changed on my car.
Definilty worth it. Only costs $25 dollars and super easy to replace; only 2 bolts. I suggest it.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)