IMO they are not good additions to hi-tech tanks, unless you just like slowly killing inverts. But at the same time, they are wonderful additions to low-tech tanks (low filtration, no UV) especially if you have a bit of Green Water (diatomic algae). Then they are in hog heaven.
In fact, with enough of them, they can be a nice UV replacement. But you have to make sure there's some organisms in the water for them to eat. And yes shalu, they do not suck mud. If they are not coming out of it, ever, it's probably because they are toast.
Steve - 33g reef and a 180g planted in need of a re-scape.