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As someone who plays with both film and digital photography the ISO sensitivity of digital cameras is almost spot on with that of film.
For astrophotography however digital and film aren't even close. Digital is quite linear for lightness changes with exposure. Film however suffers from reciprocity failure a response which is almost logarithmic.
Suppose you take a 1 second exposure on ISO100 film. To increase the brightness one stop you need to go not to 2 seconds as expected, but rather to almost 4 seconds. 1 minute becomes 8 minutes, 4 minutes becomes 40 minutes, 10 minutes becomes 2 hours etc. So at these speeds film and digital are not comparable.
That said even at faster speeds I believe there's some 4 or so different ways to measure "ISO" on digital cameras depending on which standard you follow, just like there were several different ways for film.
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