We always purchase the unlimited internet plan (there is no "card"). You just sign up on the computer with your name, cabin number, password, etc. and the fee is charged to your stateroom account. You do not have to insert any cards or anything to use the computers.
We find the plan and access to computers very helpful in keeping in touch with family back home, posting on the Dis "live" from the ship, etc. My kids love to talk to their friends so it is much cheaper for us to do the "unlimited plan" than it is to do the "minute" plan.
You have to judge for yourself if it is worth it or not. If you plan on keeping in touch with family via regular phone in your cabin, you will want to keep it short as those minute charges are $6.95 per minute.