I am basically in the same boat. After about 2 months of having a K3, I've bought one book for it from Amazon (I do have a bunch of freebies, though they are unread)
Anyway, I think it pretty much is the best e-reader around, except maybe the Sony 650, which is twice as much money. So it's still a good pick.
Calibre is sort of a pain to use. Not so much it, itself, but the fact that you have to use it at all. Then again, it has its own way of storing books that insists on using and it's kind of slow.
I have gotten most of my books from here or Manybooks or Feedbooks. So I don't bother using it for those, I just download them in mobi and stick them in a folder on the Kindle.
Still, Calibre works pretty well in converting epubs I've made with Sigil. That said, on some of the ones I've downloaded (from here for instance, the Josephine Tey books like Man in the Queue*), it seems to mismanage the left margin, so it takes up 30% of the screen instead of just a little bit. You've got to go into Sigil and delete the epub style sheet (at least that's what I do) and it fixes that.
Haven't tried PDF, some people say the Mobipocket Creator Professional (which is free, I guess from the mobi website?) does a better job with those
* Yeah, I know, I'm not in a life+50 country. But I do own them all in paperback so I can still sleep at night...
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