I only update when they have something I love instantly.
I'm using my 2009 iPod Classic/nano 5g and Apple has had two chances to get me to upgrade but plain and simple, the 2009 iPods were the pinnacle IMO to date and nothing has appealed, and I'll keep using them until something I love more comes along, which may never happen if they kill the Classic and eliminate the click wheel altogether.
I'd still be using my original late-'08 unibody MBP if it hadn't been replaced with a late '10 MBP by AppleCare. I'm on that one now, and will be for the forseeable future. At $3k for a new laptop, it's something I tend not to do that often, and I have to load it up and love it when I do get it. Oddly enough, I think I like my old 2.8 GHz C2D CTO more than this late 2010 i7; I guess because I picked the former one and not the latter, even though the latter is the better machine. *shrug*
Still using my original 24" ACD from the redesign in '08. Even paid $200 to ship it overseas by boat from Japan to America last fall. Would've been nice to upgrade, but I figured $200 was cheaper than $1000 for a new 27"er, and mine is flawless with perfect quality control and no light bleeding, so I kept it and will for some time.
Kept my iPhone 3G until the 4S came out. Wanted the 4, but oh well. Aside from the 4S, Apple hasn't made anything at all that's appealed to me since the 2009 iPods, so I've bought nothing.