Last year's trip was the first time we bought the CD at the park, and it was well worth it for us because of the sheer number of pictures and the quality of the images. In particular, the PhotoPass photographer got a great series of our girls at Pixie Hollow. Both girls were wearing sundresses with little kerchiefs on their heads, and Tinkerbell and friends went to town playing peek-a-boo with the girls and their kerchiefs. We didn't have a single one of those on our own camera because it happened so fast, but the CD gave us every last one. And then some - we had almost 150 pictures in total.
This year, we asked our TA to do some research, and she got us a promotion code for the CD. It's not a pre-purchase option, but we can order the CD when we get home through the PhotoPass site for $99. I'm guessing we'll take even more advantage of the PhotoPass photographers knowing that we're almost certainly buying the CD when we get home!
I heard a great tip here, too - bring a permanent marker so you can identify your PhotoPass card. If, heaven forbid, you drop it, at least there's a chance someone who finds it can get it back to you...