I added the plan for a month when I went to Ireland and England over the summer. I think it was 20-30.00 for 200 text messages. Paying per text could get costly quick. Also, turn off your 3G/4G for data. I would also make sure everyone has Skype on their phone if you need to talk.
Two of my daughter went to Ireland recently with their iphones.
One (4s) would work there while the other (4) would not.
It lets you make free calls and texts to others that also have the app. I used it in September and it worked like a charm (assuming of course you have wi-fi or 3G/4G access)
This past summer my wife was able to expand her coverage to cover all of Europe on a month to month basis for a very small charge. She signed up for one month, chatted away like crazy while we were across the pond and then cancelled the service when we returned.