The CEO won't go bankrupt.. these loans are backed by the stock as collateral. But there should be a law that all such plans and purchase agreements are available to all employees if available to the execs.
I had posted about this earlier here: https://news.ycombinator.com/item?id=13426949, which lead to a lively discussion. At least one founder - Steve Newman of Scalyr - came out and said that this is how his startup handles equity compensation (minus the tax cover). More people should ask and get this style of offer.
I couldn't actually find this out through a google search. What happens if the market rate does not allow the lender to recover the principle? I thought you'd still be liable for the remainder. It's just that they can force a sale if they want to.