Of course, the "neat" side if it is that then can protect you from RFID leakage from your wallet (e.g. RFID enabled credit cards being read by unscrupulous parties as you walk down the street). Popping one of these products into your wallet means that the RFID in the card can't be read until you open the wallet and/or take the card out. This kind of remind me of the practice of banks including small tyvek sleeves for their ATM cards to protect the magnetic stripe from damage -- although I haven't gotten one of these in a while, so perhaps it's no longer being done.
I wonder how long it will be before shoplifters figure out that the same devices can be used to block the RFID tags on the materials they intend to purloin.
I also wonder how long it will be before retailers, seeing the potential for loss, lobby governments to prohibit such things (hey, it's already been done in other fields such as the DMCA).