I’m not into K’nex as much now, but I have the Crazy Ball Machine (or whatever its called), and my brother has the giant snake roller coaster as well as several amusement-park ride thingies.
One thing that modern K’nex suffer is the sheer focus on license theme to sustain it’s company.
I have check some original set use bricks in the past, but it seems more prevalent on license theme.
(I have collected a few sets during my childhood.)
I have some loose k’nex pieces, but I don’t know where they came from. My guess is that they came with the big box of toys that my neighbor gave my brothers many years ago. I remember doing a project in fifth grade where we had to build cars out k’nex. That was fun (especially since my teammate let me do most of it)