These Impossible Project films are great, really good clarity and light & dark shades, provided you shield them from light immediately after having taken the photo. You can do this by putting the empty box that the film came in over the slot where the film comes out of the camera and sellotaping it in place. It might not look so hot, but it improves your picture quality no end! I had some problems with some pictures not developing, I heard about this technique, and it really works!
The alternative is to buy the official Frog Tongue accessory from the Impossible Project. This works equally well, but obviously costs some money. It also (as far as I know) doesn't work with the Polaroid One range of cameras (i.e. Polaroid One, One 600 Classic/Ultra/Pro). So the Box Method is your best bet if you're having problems with pictures not coming out.
Any technique that protects the film from being exposed to light for the first 2 minutes of its development cycle will do. The Box Method is just the cheapest and easiest way. Like I said, the actual quality of the picture is really great - in some ways better than the original Polaroid 600 film.
p.s. Not sure if I'm allowed to say this, but misskyoto is by far the best seller to buy these films from - always the best prices by miles!