I have done a lot of research on belief in free will, all of it suggesting that belief in free will is good. Of course, whether believing in free will has some advantages (e.g., people who believe in free will work harder) has nothing to do with the question of its existence.
I think one has to at least acknowledge the possibility that free will does not exist--that our "choices" are illusory.
Having a child has tipped me more in the direction of a belief in determinism. Claire's actions are a function of A) how she came into this world, and B) the reinforcements and punishments she gets from the world around her.
Luckily, she came into this world a wonderfully sweet soul, and Deb and I shower her with affection. She is a very sensitive little thing, but it wasn't her choice.
Of course we are talking about a baby, not an adult. But what's the difference? Do we suddenly cease to be a function of genetic make-up and environmental reinforcements when we turn 18? When we turn 8? Nothing magical happens when a person turns 8 or 18 or 21. (Perhaps one gains free will slowly over time, but when does that initial act of volition occur? What does it feel like? Wouldn't you know you were acting on your own for the first time?)
On the other hand, in my own life I have the sense that I am making decisions. At no time is the feeling of choosing stronger than when I am in a spiritual or ethical dilemma.
I once read a children's book (by Ursula Le Guin) in which the main character, a wizard, says that the only choice people really have is to accept or reject destiny. I think there is something profound about that answer; we either become what we were created to be or we do not.
It could be that those who prefer the light gravitate to the light--and eventually become what they were created to be; and those who prefer the darkness gravitate to the darkness and become something less. Thus free will may just be one's preference for the light or the darkness.
Whatever the answer is, I don't think it is simple.