Great experience for Panama.
Buy the Metro card ($2) at a kiosk before entering the gates. Then add money to the card at the kiosk. Simple as that. We rode the Via Argentina Route (L1) to Albrook mall. 35 cent per person one way. Fast, Clean, and Professional. It is safe to take it during rush hours in the daylight but I would recommend the off hours. We tried peak and off hours. Both are good but it is really crowed during rush hours.