In a nutshell, you can pay for the CD ahead of time if you like and get a discount, using the above link but you don't have to. The prints are expensive so if you think you'd want 8 or 10, the cd would be worth it. If you aren't that much of picture people, probably not. You can grab a photopass card from any photographer in the park and use it with any photographer. You can use more than one card if you party breaks up or a card is left in the room. There is a new Photopass Plus as well that includes ride pics and character meals. It's a new deal announced this week.
Post any additional questions you might have and I'll try to answer.
I am a picture person and I love photopass. It is like having your whole trip documented in wonderful photos. To make the CD worth it you really need to stop at the photopass photographers when you see them. The magic photos that they add just make it that much more special. The quality of the photos is amazing.