The quality really cannot be understated, they last forever. When I was growing up, my brother and I had a mix of "modern" (early 90s) kits and my dad's legos he had growing up in 1950s and 60s Germany. Now my cousins' kids have inherited both and are mixing them with 2010s kits. The 50 year old blocks fit precisely with the blocks made in the past year, and when you throw them all in a bin together you can hardly tell the difference.
It's rare to think of a toy that you can play with as "heirloom quality," but I have no doubt my kids will inherit a portion of the family set when it becomes time, and their kids just might too.