1. It's durable as hell- my rough ass 8 and 5 year olds haven't broken it yet.
2. Price is good- especially for what you get
3. Unlimited access to books/games/videos- for a small monthly fee. If you already have prime I think it may be less.
I have purchased a Nabi and while it's good, the kindle wins for the aforementioned benefits and for the simple fact that it has a common charger (micro usb). That damned Nabi has a different charger and it's a pain if you're traveling especially. Plus they short out real easy (on my 3rd one).
I won't even go into Leapfrog, VTech or any of the others, because they're simply not worth it.
Storage is an issue on the Kindle though, so be aware that you probably won't be able to keep a bunch of movies or music on it. But if you have steady wifi access, it's the best bang for your $