I have (a cheap Kobo something or other), but it was completely useless for handling most pdf documents and slow enough when navigating a document to get really annoying. It also didn't have any support for annotating or making comments. I've kind of had my eye on this: remarkable.com, but I can't quite bring myself to spend that much. What do you have/recommend?
Still not the perfect device, you can see some criticisms in the review above, but it's my favorite I've seen so far. Forgot to mention it has an SD card slot in additon to 16GB internal storage, compared to, say, the Kindle Voyage which has 2GB and that's it.
Still waiting on the dream of color eink monitors, but in the meantime this will do.
EDIT: It also has a togglable faster refresh mode, which doesn't clear the screen as well but is much easier to use for scrolling/panning around.