I just did this, bought a honda civic hybrid 2003 that supposedly will need a new battery within the next year or so. That is based on time and mileage. The honda dealer said it will run like a gas engine if the battery fails, but through my research I have learned that there is no alternator. The hybrid battery charges the regular battery, used to start the car. My understanding is that you can only drive it until the regular battery dies then you would not be able to start the car. Maybe this is specific to hondas?
So far the hybrid battery seems to be doing OK. It still charges and holds a charge, but I do a lot of highway driving.