I had some eD's edead sound deadening sheets and they stayed on the underside of my reardeck for about 5 years now and they're fine.
Do remember that when you apply these sheets you'll need a roller or something and some muscle to "mush" the sheets onto your car's body panel, if you just use hand then I bet it'll fall off within 6 months and once they fall off it loses quite a bit of tackiness and you'll have to cut it off and install new sheets.