Just thought I'd bump this up with a trial report:
Last weekend when I was in Spain I bought a Carrefour Movil card. It has several nice features such as free voicemail, and even decentish national and international rates (comparible with what most Spanish contract customers get): It costs €10 and comes with €10 of credit. A call must be made every 12 months (no need to topup though). Currently (until the end of March) they have a promotion where you topup and get a bonus. (e.g. You can buy a €40 topup voucher for only €22).
The big con: International SMS: 69c !!!!!
There was also no way I could find this out before buying the card and sending an SMS.
Other cons: You only get 5 free chances of checking your balance (via SMS) per month. After this you pay standard SMS rate.