It seems to work only on things that aren't very
stuck together.  I have only tried it using
sketches so I don't know if it works on stuck cel
layers (I assume it does).  In my experience the
sketch was not stuck that bad, but bad enough that
I could not remove it normally with no damage to
it.  When I stuck it in the freezer for a few
minutes it came off with no problems.  

I have tried it with very stuck sketches and have
had no luck.  Once the sketch seems like it is
going to tear I stop and do not try to seperate
them any further.  The only things that you should
be really worried about is that the seperation may
cause severe damage to the paint of the cels or
that the moisture from the freezer will.  I always
put my cels in the freezer with their bags on
them, and afterwards I would make sure that the
moisture is completely gone.

If the layers are *very* stuck I wouldn't get my
hopes up :(  I can't see any way to remove them
without damaging something.   I guess you have to
ask yourself if the damage is worth it.