The most important thing to remember about owning company stock is that your entire paycheck is already dependent on how well your company is doing. If your industry took a nosedive over the next six months, your ESPP may be cut in half and then you get laid off and lose your income too... Bad. News.
Rule #1 of investing is diversification, and hanging on to lots of company stock violates it. In my opinion, this only makes sense if your title starts with the word "Chief", because then the direction of the company is actually on your shoulders.
That said many of us (including me when I had a corporate gig) take advantage of company stock discount programs, in to take advantage of the very large (in terms of annualized %) and short-term gain.
Bottom line, my advice to anyone is to sell all ESPP shares the moment they become vested, and don't even think of holding them any longer than that. You can still contribute enough to maximize your gain with regard to any discount, but no more than that.
What to do with that money after... well, that's a longer discussion, and I'm sure others will chime in.
