I have "phantom" stock options that "vest" (such that they are actually worth something in the event of an acquisition) but both vested and unvested shares are forfeited if I leave the company under any circumstances. So there's one bit of anecdata for you. These were granted initially to a small handful of us near the beginning of the company and since, about a third of the original group have left, forfeiting their shares.
You get what you negotiate, I guess.