Nassarius snails are great because, unlike most other obligate sand-sifters, these can be placed into a relatively new tank since they are scavengers. As soon as you put food into the tank, they'll rise up out of the sandbed like a bunch of mini-Uboats
Excellent for keeping the tank clean in case of accidental overfeeding and this also keeps any bristle worm population from exploding since the N. snails eat up the uneaten foods.