Do the math, buying at a 15% discount, and selling immediately, is a near 17% return. I would put in the max allowed, 25k, if my company offered such a plan. However, pre tax rules the day, so put money in your 401k first, at least 6% and if you are in the 25% tax bracket, then put in more.
I would suggest selling right away, or perhaps waiting a year to take advantage of lower capital gains rates. Some years you might lose, but overall, the bias for most stocks in stable companies, is to go up.