After playing around with the PRS-650 and the Sony cover with bulit in light, I'm pretty happy. The light certainly doesn't have a natural spectrum, but it is not too bright. It puts out enough light, even on the low setting, to read comfortably with no sign of headaches for me (granted I didn't get a chance to read for very long - it was a present for my wife)! My wife read for 3 hours straight (with the rest of the room dark) and she didn't suffer any eye strain.
I'm curious to see how long the cover/light lasts though. The mechanism to turn the light on/off appears to be relatively fragile. The rest of the cover/light seems well enough made though.