I upgrade before my tech wears out. I'm careful with my ereaders so I don't have breakage as an excuse for replacement. It's usually a feature that I want that draws me to a new device, but even then, I try to wait until I see what others think about it.
I started with the Sony 505 as my first dedicated ereader - it was the Kinde 3/3g that drew me next. I was drawn to the Kindle basic because of it's light weight and the Paperwhite 2 because of the lighting. Hopefully, I'll be good with that for awhile. I can't think of any other feature that would draw me away.
In tablets (which I also use as an ereader at times) I really liked my 1st Gen Kindle Fire, but it seemed to be having more issues keeping up with web pages, etc. I finally upgraded to the Fire HD2 because I got a $60 gift card and the 16 GB model went down by $40. It was hard to say no to a $69 upgrade.
I'm very happy with the speed and enhancements of this model - it's great for reading web pages (I like fan-fiction sometimes) as well as books.
Now, phones are a different matter....