While I haven't used either the Instax or the Instax Mini formats, I did happen to see an Instax 200 camera in a local store a few months ago. The camera itself was much larger than I expected - not what I had in mind as a suitable portable instant camera, personally, so just something to be aware of. I'm guessing the Instax Mini cameras would be much more compact.
(I believe Zoey Deschanel's character in the recent movie "Yes Man" was using an Instax 200, if you want to see one in action and don't mind Jim Carrey.)