At least 80% of my reading (90% of leisure reading) is on my Kindle paperwhite. The pleasant viewscreen, font adjustability, and small, light size (while still having a larger than your average paperback screen) make it a pleasure. That's only the Kindles that are made to be book replacements. I don't at all like reading on a color tablet or computer screen.
That said, there are times I want to read a real book. Sometimes it's "cultural" I guess you could say. When I read something classic like von Clausewitz's "On War", it just seems appropriate to read it as a real book. Though I do get a lot of classics from Gutenberg in Kindle format, just because of the ease and convenience.
I will not read any kind of technical manual or image heavy material on the Kindle. Also not on the Kindle fire or any color tablets. Also rarely on the computer. I just don't like reading those things electronically. It may be just me, but I always find it a hassle to find certain areas of a manual, and often the imagery, even on a larger computer screen, just sucks. You either have to make it to small to read easily to fit on the scree, or else scroll around. Most of the time I read a manual to figure out some new thing I just bought, I will find the area of the E-manual I want, then print that out. I do that a lot these days since almost nothing comes with a paper manual anymore - just a URL to an E-manual.
Also newspapers. Newspapers suck on any Kindle. Ever since I started getting the weekly hardcopy of the Epoch Times, I have enjoyed reading a real paper on Sunday mornings. No E-newspaper I have ever read on Kindle has ever been a good experience. The formatting and navigation are just terrible. Newspapers on a computer screen, where you can read the "E-version of the print paper" are not too bad.