I am using the urinal blocks for the second winter here in north-central
Ohio.  Last year I stacked my supers 10 high on a sheet of plastic, then
placed one block on a 3" square of plastic sheeting on the top bars of the
top super.  I then pulled the plastic under the stacks, and wrapped whole
stack in plastic sheeting and stored them in a barn.  This spring, I had a
little sticky residue from the block on the plastic square.  I aired the
supers for 5 days outside before placing them on the hives.  I
cross-stacked the supers up on blocks to allow maximum air flow.  There was
no odor after 5 days of airing.  I noticed no rejection or other problems
with acceptance by the bees.    I averaged 118# per hive this summer.
 There was no taste or odor to the honey that I could associate with the
blocks.

I buy the blocks by the dozen at an institutional supply store.  I pay less
than 25% of the price I'd pay for a commercial bee supply version of
p-Dichlorobenzene.  The only difference is that the urinal blocks have some
fragrance added.  So far it hasn't caused me any problems.  I'd like to
hear from others about their use of this alternative source of
p-Dichlorobenzene.