I recently ordered a Polaroid Zip. I've had it for about two weeks now and have gone through a pack and a half of film (there are 10 sheets/pack).
Things I Like:
Paper is inexpensive, especially compared to Instax.
It doesn't use ink! The "ink" is in the paper.
The prints are stickers! I love that I can peel the back off a shot and immediately stick it in my journal.
Since the photos print without a frame, you can print two (or more) images on one sheet and cut them up. For example, I printed six (tiny) images of book covers, cut them out, and stuck the images in my journal.
The printer prints quickly.
It is rechargeable via USB! (Most Instax cameras and printers run on a CR2 battery.)
Things I Dislike:
The app is SUPER buggy. Seriously. I can't believe I didn't think to look up app reviews before purchasing because you are 100% reliant on the app to print pictures.
As mentioned in many iTunes reviews, pairing is not intuitive. The app makes it look like you should pair the printer with your phone within the app, but really, you first need to manually pair through your phone's bluetooth settings.
Cropping and resizing are buggy. For some reason, you have to click into the edit tool to see how the image will print and often, the app will automatically zoom into your image (and you can't zoom back out). It's hard to explain, but the sizing is really weird.
Even worse: sometimes, once you have the image sized/cropped the way you want, the printouts don't match what's on the screen. It will at times print correctly; other times, it will print the zoomed-in image (see below).
The print quality is meh. I don't expect much from an instant printer, but the print color IS always off — often there is a reddish tint. I know Instax prints always come out a little off, too, but I expect that and consider that part of the charm. I think it's easy to expect Zip prints to equal that of your home inkjet printer, but they don't come close. If I think of the Zip printer as a novelty item and not an actual replacement for professional photo printing, it's fine.
Even though my "dislike" list is longer than my "like" list, I don't regret my purchase. For my purposes (journaling), photo quality isn't a huge priority. The app is annoying, but it (mostly) works and I can generally figure out how to get my desired outcome. It is really, really convenient for printing mini pictures and I've loved what it's added to my journal.