We use them at work for science lessons (waves etc.) and after a couple of lessons they are usually ruined, tangled and bent so badly that they will never work properly again.
What would be great is a self-repairing Slinky. After each use just shove it in the oven and it returns to its original shape. No muss, no fuss, no messing around trying to disentangle the damned things.
Anyone wants to market something like this, my fee for the idea is half a dozen long slinkys for the school, and a couple for me to play with.