just my here.
if you are building a 100 point show car don't use it.
if you building a really nice driver car I highly recommend using it.

there are a LOT of different sound deadeners.
the type I have used and seen others use is a tar based self sticking adhesive pads, some are foil backed for heat resistance. this type works great, it's almost impossible to remove. most sound deadening materials will be difficult to remove.
in my opinion is it won't make the car worth less. it won't make your car worth more but it will make you car much easier to sell because the car will be a much nicer car to drive.