in the fall I took my old - swiss cheese exhaust off of my 82 300sd and welded up about 14 feet of 2 1/2" stainless steel pipe - I modified the path slightly for a little more straight run, but just because it was easier (I had to relief cut the inside of the curve and weld it back together - I couldn't find anyone local to bend it for me). I have been running it for about 6 months now and I am happy with the results - #1 total cost was $140, #2 the exhaust isn't any louder than it was with the swiss cheese pipe, #3 I shouldn't ever have to replace it (I used hand railing pipe - quite a bit heavier than most use) and #4 people always know I drive a diesel!
I don't notice any performance changes, or mpg increase, but all in all worth it I would say