Heh!!!
180 gal system with 16cm DSB (no Plenum) and sump with same...6 years and a few months, Prop tank system 3 yrs +, both came down without anything but ocean smells and the sand in the 180 is white below the first 3 to 4 cm (a little grey to yellow trash in the top layers)... No bad smells, no problems with cyano, although I have had a small patch o cyano now with the 180 as of late (it is being ignored a bit while I get it ready to move into the new 180 system). I have seen systems with the black spots, but those systems were usually those with heavy feeding regimens.
I have never had problems with them. I Just set up a new stony reef top biotope with 12cm bed (my only concession to the Spankster was 4cm less sand...

) and will move the 6 yr old system into a 125 lagoonal system with a dsb as well. I may use a remote DSB and settling tank for the Stony system in additiion to the DSB in it now. Prop system already has a maturing DSB that is 12 cm deep, and it will be the only system I consider removing the sand from, but that is because the whole tank is long, wide, and shallow (only 15 inches deep!)
Keep in mind that if you keep the husbandry up and export nutrients, the sand beds can have long life expectancies.
I would not have taken down the DSB in Wellford were it not for the fact that we have moved (although there have been a few probs there due to some neglect...

)