So let's say that instead of that stock just holding its own, it really is growing. Now we're talkin' real money. Just in case employees weren't going to make enough of a killing on this deal, companies often throw in a couple of extra incentives. Second, some companies don't just limit you to the stock price on the first or last day of the six month period. They may also set a base price that could last 18 months to two years into the future.
Some companies also offer a three month period, instead of the usual six months.
That effectively doubles the annual return on your money. A few companies in trouble have been known to turn ESPP programs into money-losers for their employees, so you must exercise a certain level of caution. However, some companies, usually those in trouble, may play games, either taking a couple weeks to getting around to issuing the stock or applying a black-out period to employee stock sales that just happens to cover the period in question. If you cannot sell your stock on the very day the payout is made, you lose any guarantee that, should the stock go down, you will not lose money. You may sell those certificates on the pay-out date without restriction.