As an early adopter of Impossible Project films, I have to say, yes - they're a work in progress: The black and white film is prone to streaking, fogging, fading over time, is inconsistent in its exposure and has a tonal curve like stepping off a cliff. The current color film is a bit different, in it has more acceptable tonality - if a bit too yellow in its highlights, is more consistent and less prone to fogging. But the opacifying agent takes nearly two hours to clear, so shooting this film isn't really giving you an 'instant photo' experience. As one of Impossible's 'Pioneer' users, I was able to buy a couple packs of their beta third generation color film, due out later this year, and found it much improved, both in color and clearing time. Worth waiting for.
Most medium format camera Polaroid backs, by the way, do not use these the films, but the older peel-apart pack films. Fuji still makes a 100 ISO film that is lovely, consistent and comparably cheap, at about $9 a pack.
It's lovely, to be sure, but I'm not convinced by either the touch screen interface or the tactile grip of a solid block of aluminum. After reading several reviews, it seems to me the more compelling case is for the Lumix GX7 with a Leica Summilux and Nocticron kit.
I had not heard of Saul Leiter's work prior to seeing an interview with Todd Haynes discussing his version of 'Mildred Pierce'; apparently Mr. Leiter's sense of color and framing was highly influential in Haynes' film adaptation.