I think there are several advantages of utilizing the Photopass Photographers. We used it on our last trip, and I am planning on preordering before our trip this fall. I think it is great to have pictures with all of our family together. Yes, I have a really nice camera, and yes, they will take pictures with your camera as well.
I like the special effects that they can add to your pictures after the fact - like what a PP posted, and others. I have my kids holding the balloons, Tink wings on my daughter, and a few others.
I also liked that I could go online after our trip and add various borders, character autographs, etc to the pictures. Our initial 125 or so pictures grew by a couple hundred more after I added borders/autographs/etc. I added various borders, etc to the same picture to have whatever borders I wanted to print them with at a later date. They all came on the CD(s).
You do not have to purchase the CD before or after the trip. You are able to go online and order a picture, or several, instead of the CD.
The advantage of preordering the CD is the $50 savings (about $99 when preordered vs. $149 ordering afterwards). If you preorder the CD and you are not happy with the pictures or decide it was not worth it after ther trip, you can call and get a refund of the prepurchase price.