We have three BB tanks, a 37g, a 67g and a 120g. Had them up for a year. Previously, all our tanks, ranging in size from 2.5g to 180g all had sandbeds. Would never go back to using sand as I now consider them detritus traps.
Here's a thread that, partway down, has pics of the amount of detritus we siphon out of our 67g on a weekly basis:
http://www.canreef.com/phpBB2/viewtopic.php?t=16978