When I first got one I used a neoprene sleeve (unsurprisingly they smell like wetsuits

). Then a couple of months later I took it out of the sleeve one day to find that the screen had broken. I have no idea how or when, but I had amazon call me straight away (9pm on Sunday, I think a call centre in America) and a new one arrived Tuesday free of charge, and I printed a return label to send the old one back free as well within 30 days.
At that point I decided to buy the leather cover, since the stiff cover will spread the load of any sharp impacts much better. However... there's an issue with the non-lighted covers, which is that the clip to hold it on is metal, and for some reason if the paint wears off it can short with something inside the kindle occasionally, which causes it to drain battery power and reset, losing your page.
So after experiencing this myself and then finding out what the problem was on the official kindle forum, I e-mailed them saying the cover wasn't fit for its purpose and referenced the forum threads, and they refunded the cost to me, without requiring me to send it back, so effectively I got the cover for free.

I just need to put something non-conductive on the clips to make sure it doesn't happen again (thinking nail varnish).