Color PDFs and comics almost always assume good, full gamut color reproduction. e-ink can't do that. It can only do a few pastel colors well.
The typical books usage is much more in-line with the capabilities of the display. Highlighting is substantially improved with even a little color, as are interstitial art and cover icons.
The E-Ink Spectra 6 displays [1] released last year has full color gamut, thus beating any other display technology in an outdoors scenario. The problem is that they take something like 12 seconds to refresh and I haven't seen any consumer devices using them.
In the near future I think spectra displays will be excellent for readers when they manage to get the refresh a bit more speedy, while ironically the B&W displays with color filters will be the E-Ink technology suited for tablets and phones in general.
Any color at all would be invaluable for diagrams, schematics, graphs/plots and other reference material where different subsystems, pipes/cables, datasets, etc are distinguished by color.
Will it be good enough for, say, graphs? A lot of scientific papers and news articles have illegible figures when reproduced in black and white, because you can't tell the lines apart.
Depends on the graph and newspaper. The best case scenario is a desaturated version of old print newspapers. Their DPI was near this screen's DPI (200 vs 150) and it'll suffer from the same problem of losing low contrast details. You won't be reading rendering papers, but it'll probably help a basic line graph.
The typical books usage is much more in-line with the capabilities of the display. Highlighting is substantially improved with even a little color, as are interstitial art and cover icons.