If you leave the company, you have to exercise the options (typically within 30-60 days), and pay not only the strike price but the AMT on the difference between the strike price and the latest per-share valuation (i.e., the paper value of the options). So if you have options at a strike price of $50k that are now worth $5mm, you're in for well over $1mm of taxes payable -- now -- in cash.
My understanding is that this practice is legally dubious and has been challenged, but IANAL.