I used few ebooks and Kindle is the only one that actually works as expected. Some ebooks I used drained battery in few days, not delivering promise of long life. Some ebooks were just crap and broke after few months. Kindle works few weeks from one charge for my use (1-2 hours of reading per day), it's water-proof so I can read my books while taking a bath (priceless). I never had any particular issues with it.

Its UI seems oriented to promote Amazon Store and I never used it, sending books over e-mail and deleting after read, that's OK with me. I'd prefer for its library to have folders and I'd prefer for it to work as USB stick like other ebooks do, so I can connect it to PC and organize things inside as I want, but those are not necessary.

So may be Kindle is bad, but rest are worse, I don't know a single ebook brand of Amazon scale. They all seem to be Chinese no-names which come and go without investments to quality and reputation.