I got the Voyage and the Oasis about a year ago. Before that I had two 4th generation Kindles with buttons and a 7th generation basic Kindle. I have to say that I stopped using all my older Kindles after getting the newer ones. The screens of the basics just weren't satisfying any longer. So I would guess that going from a H2O to an old Kindle could be somewhat disappointing. But with very good ambient light, especially bright daylight, the difference might not be all that huge. In daylight the older screens still look pretty good.
Also, the 4th generation Kindles are indeed noticeably slower, but that's only if you load several hundred books on them. If you keep only a handful of books on them, they're pretty snappy.
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