I had the iPad 1, 2, and 3. I wanted the extra RAM, cameras, and thinner and lighter design of the 2, and the retina display and extra RAM in the 3.
The 4 came out 6 months after the 3 did, so I was aggravated and didn't upgrade. Plus it didn't really have anything I cared about.
I would have upgraded to the Air, but it felt almost hollow and kind of cheap for some reason.
By the time the Air 2 came out, it was time to upgrade. My 3 was painfully laggy and the Air 2 felt great. Plus I wound up converting from black to white starting with my iPhone 5s, so I wanted a silver iPad. I don't imagine I'll upgrade to the next one but who knows. The next iPad that makes my Air 2 feel antiquated will probably be what I upgrade to.