Read a few reviews criticising the 'Blind' nature of the Power Discs. It's widely accepted that ANY blind bag collection will inevitably contain repeats if you purchase more than a few bags; if you don't like that, then it's extremely easy to purchase the desired individual discs seperately online for premium prices. Many people enjoy the mystery and challenge of obtaining the particular discs they want through blind bags; yes, they are expensive, but it should be taken as a given that you won't always get what you want, and shouldn't affect your review.
Now that that's over with, on to the ACTUAL review. Power Discs for Disney Infinity work by placing the Power Disc on the 'Infinity Base' obtained in the Disney Infinity Starter Pack, and work very, very well. There are multiple types of Power Disc; circular discs which power up your character, such as by increasing health or defence, and hexagonal discs. Hexagonal discs can either change the theme of the 'Toy Box' mode via alteration of the sky or terrain (such as theming the area to the 'Tulgy Wood' from Alice In Wonderland) or add items to your particular character, either by equipping unique weapons on your character or dropping a unique vehicle into the 'Toy Box'. Discs work well and immediately via NFC, and do exactly what they say on the package.
If you enjoy the thrill of collection then Power Discs bags are a great way to go. If not, and you want specific discs, then a simple Google search will allow you to identify other ways of obtaining them; although, of course, you could always just trade with a friend.