Once foresight is cast, you receive instantaneous warnings of impending danger or harm to the subject of the spell. You are never surprised or flat-footed. In addition, the spell gives you a general idea of what action you might take to best protect yourself and gives you a +2 insight bonus to AC and Reflex saves.
I read this as doing the following things:
1.) Instantaneous warnings of impending danger or harm: The DM tells you what he is planning the round before he does something if it will present a threat to the PC. Instantaneous and impending indicate to me that you should have little warning, but enough to act to save yourself: usually 1 round.
2.) Never surprised or flat-footed: self explanatory.
3.) Best action you might take: The DM gives advice on what you can do to avoid the threat that he has presented to you. This includes telling you how to deal with a foe if you have a way of dealing with it quickly.
In my game, I role a saving throw (and record the result) for any significantly threatening creature at this point and then tell the PC if the creature will fail a saving throw versus his best suited spell, staff or other item (assuming there is no other obvious better tactic available to the PC than his best spell, staff or other item). Then, I apply that saving throw role to the first saving throw the creature makes during the turn of the PC with foresight.
If the PC is going to be caught in a dragon's breath, I role up the damage and tell him how much damage he will take and suggest what I would do, if I had full knowledge of the situation, to avoid the danger.
4.) And the spell grants insight bonuses: self explanatory.
#2 and #4 are nice, but are not worth more than a 5th level spell by themselves. The juice is in parts #1 and #3. Foresight tells you limited things about the future, helping you avoid danger and threats. That is *incredibly* powerful.
For you comic/movie/old tv fans out there, think of Spiderman and his Spider Sense. Now, imagine that Spider Sense not only telling Spiderman that there is danger, but instead telling him exactly what to do to avoid the danger. Instead of those wavy black lines appearing around his head in the comics, he gets a thought bubble with a picture of what he needs to do to escape. Spider Sense on Steroids.
If your DM fails to consider your foresight, it is a very bad spell. If he follows it as written, you should be very happy with it.
A word of warning: Many DMs will find this spell to be annoying as it ruins all their careful crafting and planning. I tend to avoid this spell as a player because I think it makes the DM have less fun.