We bought 3 Net 10s from ebay before we last went and had no problems at all with them. They needed activating once we got there but it was all pretty painless and DW did it in the car on the way to the park on the first morning.
We only used them for calling or texting each other within Florida though - they didnt have the facility for facebook on them, and I dont even know if calling home was an option as we had skype on the laptop for that.
Ours came with instructions with them.
As I remember it we had to use DW UK mobile to ring the operator and activate the first phone - we then used that one to activate the other 2.