Are you talking about the pre-purchasing of the new PhotoPass Plus or just the regular PhotoPass that you can get in the park from any photopass Photographer?
If you are headed to Disneyland for 3 days or longer, this is absolutely worth your money to buy the PhotoPass Plus.
If you prepay, its only $69.99 plus taxes and shipping, and you get an amazing amount of benefits.
For the one time price of pre-purchasing the PhotoPass Plus, you are given the ability to get digital copies of all your photos taken by PhotoPass photographers in the parks (just like the normal Photo CD that you can get by using a Photopass card & the photographers in the park) but the Plus option adds the on-ride photos as well as photo packages from character dining locations.
There are two ways to get the PhotoPass Plus, either online before you arrive or in person at the Disney Parks. I advise you to pre-purchase, because its significantly cheaper. Pre-purchased Disneyland PhotoPass Plus is $69.95. In the park it will run you $99.
No you get ALL the photos that the PP photogs have taken, plus the ride & character meal photos that you have had the CM scan onto your PP Lanyard/card (they give you the lanyard when you take the certificate that they mail to you in to the parks photo shop) on either a cd or a download. We did the download and it was super easy!
For 5 days in the park we had somewhere around 200 photos or so. Its a great deal!