We've done the Photopass 2x this year. On our 7 day trip, we ended up with over 300 photos after the editing. On our 4 day trip, we still ended up with about 85 photos after editing and we really weren't looking for the PP photographers on that trip - we just didn't have that much time (we have quite a few Star Wars PP pictures from the 2nd trip though).
Even with less than 100 photos, I still think it's the greatest deal -- I added them all to my iPhoto library and now have some on my phone and of course also had them printed for way cheaper than WDW. With the price WDW charges for an 8x10, if you're planning on possibly ordering 7-8 photos from WDW, the Photopass is a better deal. And it's great to have the usual family photographer (in our case, me) in the photos as well.
Edited to add -- if your family splits up into various groups, each group can have their own Photopass card and you just add them all to your account when you get home, so you don't all have to be together to get the Photopass picture.
2nd edit (just can't stop!) -- here's a link to a map showing all the parks and where the PP people are usually located.