That's the problem with many proprietary consumer devices (take the iPad into this category as well). 1. Vendor lock: You can't install or change most of the things including OS and Apps. 2. Locked down OS: You can't do anything with your device. If you try to change the OS you'll maybe brick it. 3. Planned obsolescence built in: If the vendor decides it's "time to say goodbye" you won't get updates anymore and proba…
Sony DPT-RP1: 13" display, only displays PDFs. They added a feature in it to one of the updates that you can use it as an external monitor. Haven't tried it yet. Sony is extremely slow when adding new features and updates, it seems like their release cycles are every 2 years. The screen is I think the best on the market right now.
Sony DPT-S1: 13" display, only displays PDFs. This is the older version of the RP1. Oddly enough, it has a lot more features software wise. It has a web browser and SD card reader. Using a browser on an e-ink display is possible, but it's painful to use. It sucks typing out a URL and scrolling because of the refresh rate. It would be somewhat better if there was a way to turn through webpages instead of scrolling so the screen would only refresh once.
Onyx Boox Note 13": Onyx e-ink devices use Android (older versions of Android). The good thing about these devices are that they could display almost any type of format - PDF, Mobi, Epub, etc. The UI is somewhat clunky and could use some polish and so could some of the English translations. The updates come more frequently but it's a gamble on whether the new features are worth dealing with the new bugs they introduce. The larger versions you could also use as an external monitor as well.
Remarkable: 10.3" display. Best note taking e-ink display I've ever used. It's almost like writing on paper. I would only use it for drawing and note taking though. When transferring PDFs, it converts the files to their own format. They also have an app which allows you to sync files with your phone. Their last major software release they had to pull twice because of bugs.
Amazon Paperwhite: Nice small device which allows you to read in the dark. Most of the large e-ink displays do not have backlights. Great for reading anything that doesn't have graphics. Do not read PDFs on them.
Microsoft Surface: Not an e-ink device, but portable enough that you can read any format on it and more. Comparable in price to a lot of the 13" e-ink displays since they normally go for $500-800.
I've never found one best e-ink device out there for all uses. They all have certain things they excel at. They are all lightweight low-powered devices though, so if you want to do anything besides read or write text on them you'll probably end up being disappointed.