PU underlay does suffer compression set as with all polyurethane
foams, probably 25%. So the extra thickness does tend to diminish a
little over time (weight of carpet is not inconsiderable).
The only real downside with thick PU underlay is the "bounce" with
furniture that has large load spreading feet. In this instance the
furniture will "rock" on the underlay as it is not rigid. With
furniture that has small feet it will show deep pressure indents.
Rubber underlay has better recovery than PU and lower compression set,
so does not suffer these downsides.
The benefit of PU is actually thermal insulation, last time I looked
the really thick stuff was about 30-50% higher TOG rating which can be
really beneficial if you have solid uninsulated floors (or uninsulated
ventilated wooden floors with a howling gale blowing under them). It
can stop the "aching calves" from cold in older houses etc.
Some say use the thickest PU for bedrooms, thinner for elsewhere.
The thickest PU can lift carpet away from the grippers, but proper
stretching overcomes this - PU does soon drop back a bit after a few
Rubber waffle & crumb do perish and breakup over time, rubber crumb
CAN suffer compression set with high load - it basically turns into a
"depression" which is unpleasant to walk on and somewhat hard. So it
is not true that they outlast the carpet where someone "sits in the
same place or walks a corridor in the carpet".
Never nail wooden backed carpet grippers into asphalt covered floors,
it breaks up the surface which is often your only damp course. Not so
much a problem re damp usually, it is just a mess that is time
consuming to fix. Instead use a polyurethane spray adhesive, gripfill
or such like. Not much will stick to asphalt so you can still get them
PU underlay comes in "quiet" (no paper) and standard versions. Last I
used was Tredaire Dreamwalk for my mother's and it made a bitterly
cold floor tolerable, no "calf ache" and it gets all the cold air
"traffic" to a radiant gas fire. To reduce the drawing cold air
through the entire house (and thus chilling the floors) a colleague is
"piloting" a baffled-vent with external pre-heater. Really need a 2008
winter to test it properly though.
If your floors are cold, getting the highest TOG does help. I'm
surprised we haven't done a closed-cell polyethylene based carpet
underlay, that feels noticeably warmer to the touch than polyurethane,
not checked their thermal conductivity figures or compression set.