What is free will?
If "free will" means that we control our own actions and ultimately make our own choices, rather than anyone else, then yes, we do have free will. We have direct control of our actions and bear ultimate responsibility for them. If you want to wear dirty underpants on your head when you go to work, that is absolutely within your power to do. You are largely free to do what you damn please, the only corollary being that you must accept the consequences.
If "free will" means that it is impossible in principle to predict how someone will use that control, then no, we don't have free will. That "I" control my own actions doesn't mean "I" am an unpredictable black box. My decisions are guided by real factors, from my life experiences to my belief system to my brain chemistry to the amount of iron in my blood, and every one of those factors is, in principle, knowable. Thus, I can predict with reasonable certainty that nobody reading this is actually going to go out in public tomorrow with their underwear on their head. You could, but you won't.
On the other hand, if "free will" refers to a practical principle that one can never 100% predict what someone will or won't do, then believing in free will is certainly a valuable principle to have. The fact that our actions are predictable in principle doesn't change the fact that there are hundreds of factors involved in even simple decisions like what I'll have for breakfast this morning: everything from my household income to my geographic and temporal location to my prior diet to what's in the fridge to my family history to evolutionary imperatives conceived in response to conditions that existed millions of years before I was born is a relevant fact that can demonstrably affect the outcome, along with an unknown number of other factors I haven't thought of and an unknown number of other factors that nobody has yet thought of.
Every prediction we make (even about ourselves) is thus done with grossly incomplete information, and incomplete information never merits more than incomplete confidence. Maybe one of you has a fetish for wearing dirty underwear as a hat. Maybe one of you is so contrary by nature that since I've predicted no one will do it, they'll feel the need to do it just to prove me wrong. Not only do I not know for sure, realistically speaking I can't know for sure; finding out the necessary information would require more resources than anyone has and knowledge no one has yet discovered. In this sense and in this context, believing in "free will" is a convenient rule of thumb that allows us to maintain a crucial sense of humility in the face of perhaps the greatest challenge in scientific history: unlocking the secrets of the mind and of consciousness itself.