Novels and light non-fiction is easy to read on the Kindle but for non-fiction with lots of ideas that are new I often have to buy the hard copy as well and underline and notate. Still I like the Kindle version because I travel a lot.
As a hybrid user who reads many books in both physical and digital forms, I have found that my preference for Kindle is directly related to how much I have traveled in the last month or two.
I've been reading a non-fiction on my phone. I like how easy it is to highlight/annotate, though I haven't yet tried to go back and review my highlights...