I did a test run and sent a postcard to myself a few days before this writing. The final product was a 6-inch by 4.25-inch postcard, with my high-res photo printed right to the edges on one side ("full bleed") and the message and address typed out on the other. The photo paper itself seems of high quality, and has about the same weight and feel of a traditional postcard.
A neat bonus with the app is that every time you send a postcard to a new recipient, Postcardly creates an @postcardly.com address for them (like firstname.lastname@example.org), linked to the person's physical mailing address; the next time you want to send that person a postcard, you don't even need the app to do it, you can send from your e-mail on your computer. You just attach a picture to an e-mail, write your message in the body, and hit send -- and the postcard is automatically deducted from your total.
I did find one thing confusing with the app during the move and scale phase of the process. Once you have chosen your photo, the move and scale screen shows your image sideways if it's a landscape (not portrait) photo, using the screen as your cropping tool. While it's an elegant solution for cropping, it made me question whether my photo was going to show up sideways. Perhaps a better solution would be to have a fixed landscape sized rectangle on the screen as a guide when holding the phone in portrait orientation. This way you could move and scale your image, then move on to the next screen to fill in the message and address, while the image is always right-side up. It's not a big deal, but I think people will find it confusing on the first few attempts.
Even with this minor interface issue, Postcardly Postcards is a great idea that gives people the option to send a snail mail postcard without the need for finding a postcard, buying stamps, or walking to the mailbox. The pricing seems worth it to me for the convenience, and receiving something personal in the mail is a rarity these days, making Postcardly Postcards a treat for all the people you send them to.