ESPP -> FUBAR

[IHateKeithSmart]

So, I've screwed up my Employee Stock Purchase Plan (ESPP). My plan allows me to accrue funds for 6 months, and then do a bulk purchase of company stock for a 15% discount. If I do a same day sale, I can lock in a 17.65% gain.

Of course, that's what smart people do with these. Me, I've held a few blocks of stock, and now I have about half below market value. Going forward, I am planning on doing same day sales, as I got advice from someone who said "Only keep the stock if you would buy it outright with the same money" (i.e. don't hold it just because you got it at a discount).

Does anyone have any generalized or specific advice about ESPP? Should you always do an immediate sale? What percentage of portfolio would you feel comfortable having invested in your company's stock?