Relatively static text and certain graphics work best, and e-ink truly is the best option for these in many cases.

With e-ink, persistence is free, pixels are cheap, paints are slow, colour is (mostly) nonexistent, and the more ambient light the better.

Paginated-navigation (whole screen changes in one go), line-art, and dithered or halftoned images work relatively well.

Display power consumption is minimised so long as paints are small and/or infrequent, and it's possible for a small device to go days or even weeks between charges, though that implies reading-based use only, lights, radios (WiFi, Bluetooth), and speakers off. I find that Web browsing drains battery at roughly 10x the rate of ebook reading.

Text is beautifully crisp, approaching laserprinter quality (~200--300 dpi).

It works well for reasonably static content, whether that's a single image that remains in place for hours, days, weeks, months, years (possible with no further power), or something that tends to paint the whole screen at a time, possibly modifying a small region (e.g., a page of text whilst typing).

It's possible to do shaded images, animation, and videos, but you've got to work carefully around contrast for colour-differentiated graphics ("grey goop" isn't an existential threat, but it is a visual menace), and the trade-offs between display quality and refresh speed are fairly significant. E-ink can drive 15 Hz refresh, but that comes with ghosting and display artefacts on my Onyx BOOX (X-Mode).

At the highest quality modes, scrolling is a hot mess, and the screen dissolves into what I call "pixel soup". The worst part of this is that it's then hard to tell how far you've scrolled and to reacquire your reading or focus point when you stop. The compromise is to use high-quality when the display is static and Speed Mode (these refer to specific E-Ink display modes) as the image is updating, there's a hybrid mode which offers this. (In practice, you'll set apps that require scroll to this mode and the issue largely goes away.)

Effects which are acceptable on emissive colour displays, such as shadings, gradients, background colours, and lower-contrast or colour-differentiated text and background often fail spectacularly on e-ink. A constant frustration is graphics or visuals which use colours to differentiate data. Um, "this grey means A and this identical shade of grey means B" ... doesn't really work. If your graphics don't work when printed B&W on paper, they probably won't work on e-ink.

My Onyx offers 16 shades of greyscale, and there may be some slight differentiation, but an alternative means of differentiating data would be better. All those early Lotus 123 and Excel chart value shadings and datapoint characters suddenly make a lot of sense again.

What e-ink excels at is of course reading static, formatted, B&W / greyscale-focused documents with an emphasis on line-art or well-rendered halftone / dithered images. Those are crisp, clear, readable under any lighting conditions, and can be read even with the display or device powered off. Text-based generative tools (e.g., editors, Linux shell) work quite well.

Quite good: Notetaking, B&W/greyscale painting or drawing, audio playback (podcasts, music), voice comms (generally w/ third-party apps such as Jitsi Meet or other conferencing / voice / audio chat apps), Web browsing (so long as colour and animation aren't key).

Not so much: gaming, photography, video, high-fidelity / colour images.