I got the Instax Mini 50s because I thought that the pull-out lens of the 7s was a little random, and I heard that the flash of the 25 was not too smart. So I held out, and now...I have a 50s. I used to have some Polaroid cameras in grade school (both the color polariod and the B&W polaroid where you have to put an emulsion on the print so that it develops), so I have something for reference.
There are some good things about the new Fuji system:
(1) It has a nice clear, picture with fine grain (especially considering that it's 800 ASA film)!
(2) The camera is big, but not as big as it used to be with the polaroids.
The drawbacks:
(1) The images take a while to come up. By a while, think like a minute to see something resembling the final photo, and at least 10 minutes (I'm guessing, I checked again after an hour) to see full color saturation. The initial image looked a bit grey and washed out and low contrast, but the final image was sharp, saturated, and had clearly defined colors.
(2) The Instax mini film is pretty high contrast, even for 800 ASA. Suppose you have a diffuse light source in the room -- when you take a photo with a subject 1/2 in the light, the light gradient won't be as diffuse as it looks IRL. It will be much more marked, almost like a line. So you've been warned!
(3) The print is REALLY small. Now, it says credit card sized. That's kind of like the size of the film. There's a substantial border around the image, so it's actually smaller than a credit card. Perhaps 2/3 of the size of a credit card.
(4) You can't turn off the flash, even in landscape mode.
(5) Prints are something like...$0.70 each if you buy the twin pack (2 x 10 exposures).
I got this camera in place of a photobooth for an event -- it should work, and even with the cost of the film, it will come out to be less than a photobooth.