I have owned every iteration since the 3G, using the proceeds from selling the old generation to subsidize the purchase of the latest model. But, with the 5, I'm seriously thinking about keeping it at least until my contract is up. Hardware-wise it's everything I need. My 4S was already plenty fast, but its battery life was crap and I wanted the bigger screen and LTE of the 5. Now I have a great phone that gets great battery life. The only thing I really want is a bigger screen, but that probably won't be coming until 2014 if ever. Combine that with the fact that it is getting harder and harder to jailbreak iOS, and I think I will probably stick with my 5 until there is a compelling reason to upgrade (faster processor and better camera sensor will not cut it).
So, yes, I have historically bought every iteration. But, that will probably not be the case next time around.