I downgraded from the 700 to the 505. I was starting to see many, many dust spots which rear their ugly heads when using the light.
I also found I just was not comfortable reading on the 700 unless i always used a light. So, my battery life was lacking.
I am thrilled with screen clarity and battery life of the 505 and am happy to have switched. Im sure the 600 would be a treat.
By the way, i gave the 700 to a gal at work would never be able to buy one for herself (single mom). She LOVES it, and reads with no light all the time. Go figure