I have three devices: a Kindle Keyboard, an iPod touch and a Playbook tablet.
I read on all of them but all things being equal, I prefer the kindle because it is lighter. When reading on the tablet the weight gets uncomfortable for me after about an hour when reading one-handed, and my eyesight is not the same it was twenty years ago, so reading on the iPod with a suitable font size makes me turn pages frequently.
None of those factors is a deal-breaker mind you, I could easily make do with just the Playbook tablet or just the iPod, but I've grown accustomed to the kindle, for me it's the closest thing to a book in terms of having it around me at any time because it weights less than a thin paperback, reading comfortably in bright lighting conditions (but not direct sunlight, I don't like reading while exposed to bright sunlight
), and forgetting about rechargers.
About the batteries, I initially had the kindle and the ipod, and used the ipod quite heavily to store contacts, receive e-mail, play games, internet access, reading and listening to music(of course!). As a result I had to recharge it daily and after about two years I had to have its battery replaced because it was holding just about three hours charge for moderate use. About the same time I bought the tablet in a sale, and it took over almost all the functions of the ipod, only leaving it to listen to music or reading in poor light. I honestly can't tell how long the kindle battery lasts now because I usually recharge it once a month when loading new books and only once have had it run out on me. As I said, for me it's not a deciding factor, but it is a major convenience nonetheless.