I agree with knc1's long statement above.
I'm using my Kindles just as a hardware platform to read
free ebooks (mainly classics from Project Gutenberg and (translations of) Greek and Latin ancient texts). Most of them are in epub format; but interestingly I still have some very old prc files from my very first ebook reader: a Palm Pilot (bought exactly 20 years ago, and it's still working...
Why reformat them, as long they are still natively supported?!?
Though I have an Amazon account (to buy e.g. DVDs and tools, etc.), I've never ever bought an ebook from them. And yes, I even never had connected my Kindles to the Internet at all. Just pulled them out of their boxes, created the update blocker dir (just to make completely sure...) and then did the jb procedures (the update to a more current Amazon basic fw - I still prefer v5.8.11 for various reasons! - was made via side loaded *.bin files, not online of course).
My "everyday's burner" for reading is, BTW,
Vlasovsoft's CoolReader, because this reader is highly configurable, especially the margins and tap zones; and the feature I like most is the cool and also highly configurable status line on top of the page (in my opinion much, much better than the rudimentary status info in the bottom of the screen in Amazon's firmware, as it also displays the time and battery percentage).
I also tested KOReader, but didn't like it. Too bloated. To difficult to configure. This is mainly because I don't have any PDF books that could take advantage of KOReader's really very good PDF reflow feature - my PDF files are just scanned(!) books, i.e. bitmap files; and for them, interestingly, the Kindle original firmware is the best to read (especially because of the contrast setting feature).
Another feature I like very much in Vlasovsoft's CoolReader are the user defined sleep/screensaver files. As my epubs often don't have a proper cover picture in them, I even wouldn't like them to be used as sleep screen. I'm very often travelling by train, and it's of no interest for other passengers around me to know what I'm actually reading. So I have some mere generic 16 color grayscale pictures of my personal choice, being randomly used as screen savers. This is in my opinion one of the most fascinating features of CoolReader.
Without the possibility to jailbreak, I'd never buy a Kindle.