Counter-sue for damages consisting of the total value of my foregone compensation package (which are typically 2-4 year packages). Triple real damages because enforcing a prime facie unenforceable contract is an unfair and deceptive business practice. I would be more than happy to be the test case for this novel theory.

My strong suspicion is that no competent employer has the balls to do anything other than settle the counter-suit (less about the $ amount than the precedent), and my even stronger suspicion is that I'd win the triple damages if the poor sops in legal were desperate enough for a severance package to try and fight.