Product review (4 stars): Easy enough to fill and cheaper than buying the whole filter
As other have pointed out, it can be messy filling the filter but they provide a piece of styrofoam to put in the center hole so the granules don't fall all over the place.  The foam does break easy though - in which case, a couple of paper towels stuffed inside the center hole will also prevent the granules from spilling everywhere.  Definitely worth saving the money though over buying the new filters.