I have 2 opinions on ESPP plans:
1) Using an ESPP to buy discounted shares is by far the best risk-free return in the world. If you have a 6-month contribution cycle, you get an instant 10% gain on money that (on average) you had invested for 3 months*... instant 40% rate of return.
2) You should never, ever own shares of your own company, because you are already invested in it to the tune of your entire salary. If your company gets into trouble (say, accounting irregularities, etc.) your job may be in jeopardy at the very moment your investment goes "poof".
I reconcile these by diverting the maximum into my company's ESPP, and then selling the shares within minutes of them being deposited in my broker account. You have to check the details of your plan though. Mine comes with no holding restrictions, but many require minimum holding periods. Being short the stock in a separate account is another option, but this can be expensive over time and some companies have policies against employees being short the company's shares.
*Your first contribution is in there for about 6 months, the last one for about one week.