On the other hand I have absolutely no doubt that when a new iMac comes out that my year old iMac will quickly fetch 50-60% of its brand new purchase price (if not more) allowing me to upgrade to the new model for a reasonable price.
It could fetch that on the only market, but your family isn't going to let it go to the open market, and they'll want mates rates to boot.
The iMac may have a productive life to me of 2-3years yet they keep ticking along in the extended family for 5-8 years after.