Got the back yesterday and took it out for a spin today. Here my observations:
1) It's a lot, lot of fun!
2) The image area is not fully covered as expected. I knew it (and chose black background instax film for this reason), it disturbs me even less than I thought.
3) ISO 800 is at the limit, or even over, of what a Hasselblad can do with 1/500 at f/22 in daylight conditions. You'll need a 2-3 stops neutral density filter if shooting in the sun.
4) Instax is low-fi. A Velvia it ain't. But that's part of the fun I guess.
5) The film has very low exposure latitude. See example below.
6) Did I say it was fun?
Here my very first photo. I told my wife I wanted to take her portrait, but did not tell her about the Instax thingy. Seeing her picture immediately was a nice surprise to her. Her reaction was, "Your new gadget is fun, but if I had known, I would have dressed differently"
I exposed for the foreground. See how overexposed the background is.