[JoeD]

I don't have a set criteria, there just has to be a sufficient number of new features that I really want although I do try to get 5+ years out of devices I buy, with exceptions.

Originally I read on a Palm Tungsten C for about 5 years. I replaced that with a Hanlin for the larger screen, eInk display (for outdoor reading) and longer battery life. Easy choice even though the Hanlin was expensive. Whilst I'd replace it in a heartbeat if it broke, the cost was too high to justify upgrading whilst it still worked.

The upgrade to the kindle however turned out to be just a small feature jump, I wanted the wifi support and dictionary features but wasn't willing to pay a lot to get them as the Hanlin was/is still perfectly usable for me. The upgrade came when the kindle could be had for £50 from the USA. It hit the sweet spot price wise to make it attractive to upgrade from a still working/usable device for just a few features.

iPad I bought for work and despite the iPad 2 and 3 looking nice and having better specs, there's nothing about them that is a big enough feature upgrade to justify the cost. Not sure what hardware change would be needed for me to really want to upgrade at the current price structure. A new killer HW change not withstanding, it's more likely to be apps requiring the latest hardware to run reasonably, but it'll probably be another couple of revisions before that's an issue (there's a few apps now like that but none I have an interest in).