I have ended up with a number of the plastic frame/foundation units, and
find them time to time while going through the hives..  The nice thing
about them is there is more comb area compared to wooden frames, since the
plastic forming the top, bottom and side bars is very thin.
 
A problem I encounter is that these "frames" are somewhat flexible.  When
one tries to pry ("prise" for you British folks) the frames apart the
plastic bends and the comb has a tendency to twist.  This is not a major
problem, however.  Perhaps newer ones have been reinforced. (?)  The ones I
have were purchased by another beekeeper back in the early 70's.  I'm not
sure if Pierco (or whoever made them) originally coated the plastic
"foundation" with beeswax, or if that is even necessary to get the bees
working it.  One thing is certain, one of these deep plastic frames (or
medium for that matter) holds a lot of honey!   -- Just because of the
larger comb area available. Plus the combs are extremely even and absent of
dronce cells.  The bees are also more reluctant to goo-up the plastic
frames with propolis.
 
Acceptance of the plastic foundation, when new, is something about which I
hear very mixed stories.  Some people say that the bees ignore it if there
is wax foundation in the hive.  Others have had the bees take right to
them.  I was fortunate as the plastic ones I have were already drawn when I
got them!  SO, what is the general concensus on acceptance and comb
construction with these frames?         Tnx,     JWG