I agree- photopass is great especially if you are usually the photographer in your group. You do not have to buy in advance, but it will save you money if you do. The first park you are at you can get a card from a PP photog, he or she will take your picture and scan the card. then you give the card to each photopass photographer you encounter to scan throughout your trip. You go online to claim your photos and enter your photopass card number to see the pictures. If you forget your card you can always get a new one and the pictures from several different cards can be combined into one account. If you don't purchase in advance, be sure to buy shortly after your trip. The photos expire after a certain time- 30 days I think.
Most photopass photographers will happily take a photo with your camera as well so you don't have to buy it to get great pictures. However, nighttime shots etc usually come out better from the pp photog's camera.
We have used it every trip and love it!