I'm happy enough with the kindle 4NT I have, but I'm glad new models are continuing to be developed and released. Whilst I'd never upgrade just to have lighting added to an eink display, if my device breaks and I buy a new kindle it'll be a nice extra feature
I "upgraded" from a Hanlin because the additional features and reduced cost made it attractive. Had the kindle 4NT not been £50 I'd have continued with the hanlin as none of the features were "killers". But, for £50 to get Wifi support, dictionary and reduced weight + screen improvement over the hanlin, it was worth it. Plus I like the ease of buying a book on Amazon and having it auto delivered to the kindle, even though I'll later plug in to free the book
Not sure what it'll take to get me to upgrade again though. Whilst I'd love to see colour eInk like devices for a low cost, I'm more interested in those displays on laptops/tablets than my reader. Mono is fine for the books I read. It'll likely be an as yet unknown killer feature or more likely a combination of features + price that makes an upgrade attractive again in the future (or death of current reader
In a way that's why I'm still perfectly happy with my iPad 1 despite the speed increases the iPad 2 brings or the new screen the iPad 3 brings. For my current usage there's not enough new features to warrant an upgrade at the current asking price.
One trap I hope we don't fall into is upgrading HW to get new SW features when the existing HW would be perfectly capable of running the same SW but the company making it decides to use it as a way to sell new devices. Not pointing fingers anywhere there, just a general observation.