Whenever I had to sign one after a lay-off I checked with lawyer friends first. They are not really enforceable.
Can you explain what you mean by "really enforceable"? Does it mean that they often don't get enforced, but are legally binding -- or does it mean they're illegal?
If they're legally binding then that's still bad.
you can not waive your first amendment rights. inalienable rights can't be alienated by yourself either.