I would strongly advise everyone to max out their ESPP contributions (you can contribute a maximum of 15% of your salary, up to the IRS mandated $25,000 limit). Why? Because it's a amazing investment that, if you sell stock immediately on reception, is risk free. I'm going to assume you know how ESPPs work, google them if you don't. Let's do the math: consider a typical ESPP (yours may vary) with a 3 month purchasing…
First rule of finances: When comparing options, never compare rates. Compare absolute amounts (i.e. Dollars).
Yes, it's a great rate, but there are other investment schemes that can make you a lot more money at a lower rate.
(Of course, I participate in ESPP and am not trying to dissuade anyone).