The biggest difference in my experience (from LF, MF, and 35mm to digital) is that with film, resolution decreases at higher ISO (large chuncks of chemicals i.e. grain).
With digital, the pixels remain the same size for all iso's (its the same sensor). Thus resolution remains the same.
But since the signal needs to be amplified (as mention earlier by another poster), the S/R ratio gets worse. As a result, some noise appears, in the form of 'jitter' in what should be 'even' areas.
The good news is, that noise can be countered (up to a level of course) by software reasonably well. Additionally, with the latest sensors, the S/R are quite good, starting with a better image is always the best way of course.
So (concerning high iso) digital has the advantage of (A) not losing resolution, and ( the 'grain' (noise) can be 'processed' easier to be less objectionable (something harder with film grain within a traditional process).
To quantify by purely personal subjective evaluation, i'd say that digital has a 2 stop advantage with iso as for usability of the shots (I now have 1600 shots enlarged that now look what I was used to with 400 film, 400 digital shots look like 100 film did).