I'm a fan of Impossible Project but for me the biggest drawback is the fact that with 30-minute developing times the photos are not truly instant. It sounds like that may change soon, via twitter:
Pioneers will soon have an exclusive opportunity to try out Impossible's Generation 2.0 Colour 600 film, with 2-minutes image emergence...
They are calling this the "biggest advance for Impossible's R&D since the introduction of the Colour Protection film" and expect public release later this year.
Good news. This will make Impossible film much more useful, especially in social settings. So far the one-hour color photos have been very frustrating, making exposure compensation quite impractical. The 5-10 minute B&W development time hasn't been so bad.