Mandatory gotcha counterpoint: it's risk free assuming immediate execution, which is not the reality. Between the time ESPP is finished at price X and the time your shares actually land on your account in Etrade / other broker so you can sell, there are several days of processing time, during which the stock can easily go down 10%+ in the "current economic climate". Then, the upside might be taxable (depending on your country) and suddenly the 15% becomes 0% or negative.

(Source: saw it happen.)

The good thing though is that you may withdraw all the money until a few days before offerring time end. Useful in situations of uncertainty like here.