Let me expand a little on what PoohsHubby said....
The way the photopass works is, you have a choice about how you view and purchase the pictures. You can view them at the various viewing stations in the theme parks, which almost always have long lines and you end up VERY crowded around the monitors trying to look at your pictures, especially if you have a larger group trying to look. And then, you are trying to decide within a few minutes all the purchases you want to make. You can buy a cd there too, but you don't get to do anything special to the cd.
Or, you can create an account on-line, and register your photopass card on the account, and look at it at your leisure, and decide what to buy. You have 30 days from the time you register a photopass card - that's how long the pictures will be on-line.
From here on out, I'm only going to address the on-line option, because I haven't really dealt with doing things in the park, because it's just too limited that way.
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Now, there are some other choices you can make with photopass. Sure, you can just buy prints only from Disney - but generally, Disney's print prices are expensive. You can get as good a print quality from the prints if you buy the high resolution disk and take it to somewhere that has photo printers. If you want VERY good quality prints, you might want to go to somewhere like a Ritz camera or a place that is dedicated to just photo printing. It will be more expensive than you could find elsewhere, but should still be cheaper than the prints from Disney. If you want more than just a couple of prints, it ends up being worth it to buy the cds and go somewhere to get prints.
If you don't need the best quality prints, go to Wal-Mart or Sam's Club, or, if you have a printer that can handle printing good enough quality photos, well, you'll know that already.
As far as buying the cd - if you wait until after your trip to buy the cd, the cost is $124.95 plus shipping. If you pre-purchase the cd, the cost is $99.95 with no shipping. To get the pre-purchase, go to this link:
http://www.disneyphotopass.com/previsitcdplan.aspx
The really good news with this is that it comes with a satisfaction guarantee return policy. The way they do the pre-purchase is that they give you a promotion code to use when you go to buy your cd after the trip. If you have done the pre-buy, you have 2 weeks after you return to decide if you are satisfied with the cd you are buying (i.e., review all the pictures that will be on the cd, and make sure you think it's really worth the money.) If you are not satisfied, you can "return" the cd for a full refund - as long as you have not already used the promotion code they sent you. In other words, review those pictures carefully right away, and make your decision within 2 weeks. DO NOT buy the cd using the promotion code until you are sure you want it. If you decide it isn't worth $100, let them know, and they will give you a full refund. Otherwise, go ahead and purchase the cd, using the promotion code they e-mailed you, and they will credit you for the full purchase price of the cd and for the shipping on the purchase order.
The thing that isn't clear on the page they have about this deal is the language about the timing. For one thing, it says that to pre-buy the cd, you need to do it "before your trip". It doesn't really give any hard and fast dates. I have not actually done this deal yet - I'm just giving you the information that I've gathered together from my reading. How they determine if you're eligible or not could depend. My guess would be that as long as you pre-buy the cd sometime before you have your first picture taken on a photopass, you are eligible for the offer. That's just a guess though. I know they record all kinds of information on the photopass card, certainly time and date stamp of when the picture was taken at the least, and since many people do not stay at Disney resorts, that would be the only way they have of measuring when your trip "started". On the other hand, it could be that you could do the pre-buy just before you actually buy the cd. But I don't think I would risk it.
The other timing language that's unclear is the 2 week timeframe for the satisfaction guarantee. I mean, if you are on a two-week Disney vacation, and you put your Photopass in the first day of your trip, does that mean your two-weeks are up before your trip is over? Or is it two weeks from the last Photopass card you enter? Or two weeks from the last Photopass shot that goes into that account? EEK?!?! Not clear at all.
The way I figure to handle it, just to be sure... buy the pre-paid cd sometime before the first day in the parks, so that I've bought it before the first picture is taken on a photopass cd. And then... well, in my case, I will have a brother with NO patience on part of the trip, and he will be leaving before I do. The whole family will probably be pooling all of our photopass pictures to put on a single cd, and I doubt he will be able to wait until I get home almost a week later so he can get on-line to see the pictures. He will have his computer and internet access, so he will probably feel the need to put the pictures on-line right away. Fortunately, our trip won't be two weeks long, so we will have at least a week after the honeymoon is over to look at the pictures and make a decision. Plus, with it being our honeymoon, and given that we will be grouping pictures for not only DF and myself, but also my brother and SIL and their 2 kids, sister and BIL, and my parents, the cost of the disk will be negligible, and it only makes sense to get it for us.
Another point - as other people have mentioned, the cost of the disk is the same, no matter how many pictures you take, so take advantage. Grab those Disney photographers, and get as many pictures on those photopasses as you can.
And, one final (I think?!?) point to remember - when you purchase a cd, you can only include pictures that are in one photopass account. And each photopass card can only be added into a single photopass account. But you can add as many photopass cards into a photopass account as you want.
What does this mean? Well, you create an account at the Disney Photopass site. Once you do that, you will enter your Photopass card number into it to get the photos associated with that card loaded into the account. If you have multiple cards, you can enter each of the cards into that same account, and all the pictures, no matter how many cards you have, can be purchased for one price. As I said before, my whole family groups together to do this, making the whole thing much more economical for us. We decided to do this the last time we all vacationed together when we only had about 20 photopass pictures, because when we went through the pictures, between all of us, we were going to buy enough prints to pay for the cd about twice or thrice over. It just made more sense to split the cost of the cd and then get prints made somewhere cheaper. My home printer was good enough for the printes I wanted. And actually, for most things, I really only want the digital image anyway.
Ok, that was a long post. But hopefully, that answers any questions you might have about this. I think it exhausts what I know so far. After I've actually bought one under this discount program, I might know a little more, and maybe someone else has a better idea about some of the timing issues, such as how long before you have to buy the cd and when the 2 week period for being able to return the pre-paid cd purchase starts.