I like Koreader and have it on most of my devices, including my PC, tablet and phone. But also of course my eInk devices.
The things I like are:
- I can set up a consistent look and feel on different devices
- navigation and controls is also consistent
- customizable status bar
- supports OPDS, which makes it easy to get books from my Calibre library via COPS
- supports Kosync so I can sync progress across devices
Now to be fair, a lot of this comes from having different devices and wanting the same/similar features. e.g. if you only have Kindles you can use Whispersync and Kosync doesn't add anything.
Also COPS has a web interface which the device's browser can use to download from, although I find OPDS easier.