My only preference is to get cards that work only in one specific country, then buy another in the next country.
I usually make one call home a day for about ten minutes and it probably works out to $2 for the call. I also call around for lodging so I do use up the cards.
I also get the smaller denomination card. I haven't had a dud yet, but others have said they have had them.
I've found many phones that don't work, sometimes they don't work with a particular card and other times the phones just don't work at all. If that happens, assume it's the phone and not the card.
When I buy a card I ask where a phone that I can use is located. Usually there is one within eyesight. I'll go over and make a call right away. During the call, I compare the card to the phone to see if there are any network logos to help me know which phones work with which cards.