Any new product is going to have its fair share of negative reviews. Some of them are legitimate, some of them are just complaining for the sake of complaining. Personally, mine works great. That's no guarantee that everyone will like it or that every Paperwhite will work the same way. But in my experience, the Paperwhite works very well.
Some of the negative reviews I read complained about discoloration, uneven lighting and dust between the screen and the backlight, but mine hasn't had any of those problems.