I bought an iPad 1 and then upgraded to iPad 2 soon after its release.
However, I've never upgraded since.
This is because I don't actually use my iPad for anything demanding or important (which is contrary to my initial expectations, and is unlike my laptop and phone). All I use it for is reading, web browsing, watching videos, and a few apps. So I have no plans to upgrade.
However, if Apple ever add an active digitiser to the iPad, I'll upgrade immediately and give it another go as a productivity tool.