We have both a 500 and a 505 in our household. I agree the 505 is merely evolutionary, and both are fine devices. I prefer the 505 a bit because of the improved contrast. (I'm often reading in less-than-optimal light, and my eyes aren't as young as they once were!) Apart from that difference and the slightly greater thickness of the 500 (which isn't much), they are both great for reading lots and lots of books.
I still see a lot of support comments and questions for the 500. Simply placing the model number in the title should be sufficient. I don't see a need for a new support forum for each model. (Besides, most problems for either model can be solved by an overnight charge from the wall outlet!
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Oh yeah. And the 505 is easily available for under $300, and has been for a while. I like them both-- there's no need to 'upgrade' from the 500 if it's working.