My installation experience for the busdepot/justkamper flaps
is basically the same as others posted.  It is necessary to drill
for the front flaps.  Went right through the luxuriously applied
undercoat (which in my case is like new). I like the nut/jamb nut
set up.  The clips are a little funky.  I put them on anyway to see
how they  perform.  A second set of clips were supplied that have
no apparent application.  They are easily malleable.  I chucked
them.
On the rear flaps, the mounting hardware seemed flimsy.  The
bracket is about much like plumbers' tape (bend it around a pipe
to suspend from joist, etc.).  But, after second thoughts, I decided it
was actually a very good material for the job.  If the flap gets
hung up on a rock, the bracket will bend long before causing
damage to the fender it's mounted to.  I did drill the bracket and
the fender, and through bolted it to the lower flange of the fender.
Overall, I am pleased.  If you like flaps either for appearance or
protection, you probably will be also.  I think mine will greatly
reduce chipped paint and cut down substantially on gunk
being deposited on the undercarriage.
Traffic forced me to go through pool of what looked like 20 gals
of spilled paint in an intersection yesterday and the flaps stopped
almost all of it.  Made big points with me.

FWIW I decided not to mount one directly in front of the existing
OE flap that (effectively) protects the propane tank.

Fred 89 Westy