If you're going with kids, it's well worth it to get one of the photo packages. That way you can wait for character meets and purchase professionally done photos of them.
If you're going childless, though, it's better to peruse the selection during your cruise and pick out one or two you'll purchase. When we went, it was without young kids, so we had much fewer choices once we went into Shutters to purchase. We ended up only buying one 8x10 of the night we went to Lumiere's (we were dressed up).
Cast members onboard were ALWAYS willing to stop what they were doing to snap a photo or two from my own camera. I keep a digital camera on the ship, so I can erase anything I don't want. (I brought a waterproof disposable for the CC day, though, so I didn't ruin my own camera)
As with anything theme-park-related, your photos will be rather expensive, so plan accordingly and try not to go overboard with the pictures!
Danielle