Even when you have it in writing though things can go bad. For example, consider the recent and instantly legendary situation in 2011's business news about Zynga's discovering they would revoke some of their employee's previously granted stock option grants because their CEO was smoking crack (metaphorically) one evening and concluded that a key member of the Google team who worked there years ago (and importantly, who has nothing to do with Zynga at all) was over paid. This case demonstrates well just how badly incentive systems can go wrong. That situation was so distasteful that everyone in industry with talent knows to stay away from Zynga now, and as a result, even their IPO value went splat. Incentive systems can kill a company when they are controlled by psychopaths.
I had to fight very hard to get one at all for my mid year (financial year) performance review. Apparently that was at the expense of my peers due to there being a collective pot. I voted based on that.
In Startupland, bonuses just don't really fit in. I don't mind it too much.