I have the Sony lighted covers for both my 650 and 350, and I love them! I'm able to read in the living room while others are watching TV without having to have a super-bright lamp blasting reflections on their TV show. (I need a lot of light due to an eye condition.) And since I take the 350 in my bag when traveling, I'm prepared for whatever lighting situation might come up, in train, plane, or automobile.
Oh, and they're both totally bendable, lying flat and comfortable in the hand. I tried 3 different m-edge covers with my 600 and a nook I had for a while... way too heavy and would not fold flat behind the reader. Not having the built-in light was a pain too... always having to dig through a bag for it...