You're contributing after tax money to the ESPP so you would only pay capital gains on the money above and beyond your contributions. So, you won't actually make 15% but even at the highest tax brackets, you're only paying 37% of the difference to taxes.

I max my ESPP out and sell it immediately and have been doing that for the past 6 years. It's even better if there is a look back period and the stock has gone up appreciably.