I think PhotoPass is definitely worthwhile for a few reasons.
Mainly I like the "magic shots" (Stitch popping up out of the ground, Tinkerbell in your hand, battling Captain Hook or Darth Vader, pointing up at Mickey & friends hang gliding overhead, etc.) - you will not be able to get those by having a PhotoPass photographer use your own camera to take pictures.
I also like going onto the PhotoPass website and applying all the different Disney borders. I like being able to turn a few photos into many photos - if you come away from your trip with, say, 150 pictures taken, you can turn that 150 into 500 or 1000 or more photos just by playing with the different borders and effects and saving each one as a copy. This may be a good idea if, say, you wanted to send each member in your group a different version of the same photo - you could apply different borders
Plus, I think PhotoPass is a great back-up in case something goes wrong with your own camera, memory card, battery, film, whatever. Since it is totally free to have the photos taken, and it is even free to share them (via e-mail) with friends and family from the PhotoPass site, it's a valuable tool to use as a safety net in case something happens to your camera or pictures.
You don't have to buy a single thing. You have 30 days from when the first photo is taken to create your online PhotoPass account and view your photos. After you create your PhotoPass account, you then have another 30 days to edit the photos, share them, and simply decide if you want to buy anything or not.
I always opt to by the CD rather than any other products. I usually find some sort of discount to apply to the CD, like $15 off (always check the PhotoPass site on Facebook, as sometimes they offer special promos for Facebook users and such). The shipping charge (for U.S. deliveries) is very cheap, like less than $3 last time I ordered. So I end up paying under $50 for hundreds or thousands of photos, which I think is a great deal.
The quality of the photos varies between photographers. I've had some really great, hilarious pictures taken - and sometimes that one amazing photo is worth ordering! Some of them have been beautifully cropped and well-lit. And I've also had some clunkers in the mix here and there, too - some shots that were a little blurry.
Some of the photographers are really great in having you do many silly poses so you will have a wide array of wacky pictures and magic shots. They may have you "look scared," or "pretend you are getting run over by a train," or "kick up your legs like the Rockettes," or "pretend you're sleeping," or "put your hands on your hips," or "do a Charlie's Angels pose," etc. Others will hurry you along. The PP photographers who are stationed with the characters tend to rush more because there is always a long line of people waiting for their own pictures. But sometimes you might find a random PP person standing by themselves in DCA or something, and they are bored so they will take extra time with you and get like 10 shots!