Easy to use and does work to block credit cards being scanned when the card is directly in line with other cards above the scanner. In my trials it does NOT work if card is just beside the credit card. Ie. From my experience if you have a purse with two rows of cards side by side you need a card for each row, the card needs a bit of overlap to block. As long as the cards are overlapped 5cm of distance between a credit card and the security card will still result in blocking.
I also found that it will not block security cards though (the kind you get for buildings or room keys).
I have not yet tried for passports though based on my credit card experience I think a certain amount of alignment is required for it to block so it may not be suitable.