The reason you'd buy this is to keep the radio spectrum within your room yours. Like someone else said - this would open up more channels to you and prevent the guy downstairs running a microwave or buying a cordless phone from screwing with your signal.
Obviously you'd want to know dB shielding figures at various freqs, which you're unlikely to get from a crappy mass market article.
This kind of wallpaper is already commonly available. Also door seals, window treatments, etc.
It would be kind of worthless for normal, residential use (or even a regular office) since it would likely kill cellular bands (any badly installed wallpaper to screen 2.4GHz will hit 1.9GHz I'm sure). Plus, expensive.
I found out a while ago that in Sweden (IIRC, could have been another scandinavian place) the government pays for this for people who claim to be allergic to radio waves or whatever the hell it is this week.
Apparently it's cheaper than doctor's appointments, therapy and unending arguments, and mostly makes people happy. Usually their 'symptoms' disappear too.
If its just some dumb kid guessed your 'password' or aircracked it, the upgrade to a better WPA.
If you're being trolled by a leet hacker, then you've got bigger problems.
But please don't buy into that BS.
Oh, it costs more.
Anyone living in areas of high population density can tell you how difficult it is to find an uncluttered slice of 2.4 GHz spectrum. It's next to impossible in places like Manhattan. I don't know just how effective this wallpaper is, but if it attenuates outside signals by even 50%, it would be a huge benefit.
EDIT: Another side benefit for businesses (even small ones) is control over what networks are available inside your office. Many businesses are in office complexes where WiFi signals overlap in to adjacent suites. I can't tell you how many times I've been to offices where users are connected to a neighbor's wide-open Wi-Fi network because they were ignorant of what network they should connect to.
3G and 4G have similar frequencies to wi-fi.
Ok, so people will still buy from Amazon, but they'll have to leave Best Buy first.