I have converted old roll film type Polariods to shoot 70mm portra C-41 film. You get about 70mm x120 or so mm negs, that are great to contact print. No it is not instant, but it is a lot of fun to shoot with these old cameras.
I have the Instax 210, and I echo the contrasty prints statement, but once you know thats how it looks , you choose your subjects accordingly. Do go for the wide instax system; the instax mini prints while cute are really too small to really appreciate their content.