I just want to say one "little" thing here, but in many ways I am going to avoid the actual "free will" debate, for now...is that a choice or not?
I am on the road to becoming a "scientist" of sorts, in the field of Psychology. Along the road, I have learned something about science, that very few scientists wish to admit. They are as dogmatic about science as a person who is a religious fundamentalist can be. There is an arrogance about their method because it has brought forth so much technology and innovation.
I have heard all the defenses, including the "yes, but we are willing to throw out a bad theory or proposition when evidence is produced that contradicts it". This sounds great on paper, but is actually practiced with much resistance. Why? It is human nature to hang on to what one believes in.
In my view, and this is just my view, the illusion lives in the claim of "objectivity". The scientific method is good, but it is not necessarily objective. Scientists, like all of us, have goals. Those goals often bring tremendous pressure to succeed (financially, prestige, tenure...etc). Those pressures have an effect on the observer, just like the observer has an effect on the nature of a particle (be it human, or animal). Replication is offered as a defense against this, but the effect on the experiment is, like the original experiment, very much subject to what the researcher wants to find.
Another artifact of modern science is to create a need for a "discovery" that was not intended. This has been the dominant trend in psychopharmacology since the late 1800's. Viagra was created to help with heart disease. When it failed at that job, but was found to have a potentially valuable side effect (sexual arousal), the syndrome of Erectile Dysfunction was born. The same happened with "Restless Leg Syndrome". When seratonin was found to have an effect on brain chemistry that seemed to lift people's mood and states of consciousness, a relatively rarely diagnosed psychological dysfunction known as depression became mainstream. By the literature produced today, one would think that everyone was depressed and it was reaching epidemic proportions...and this propaganda, presented as "fact", is supported by the APA, AMA, and the WHO to name just a few.
Interestingly, antidepressant medications rarely outperform placebos in clinical trials. What is more, there is still NO solid understanding of what they do, other than make someone feel good, which can be accomplished by many drugs (legal and illegal) or activities. Yet, science marches on telling us that we are all a little depressed and antidepressants offer a solution...one would be hard pressed to not be diagnosed as having some kind of mood disorder (dysphoria, Minor Depression, Major Depressive Disorder, Generalized Anxiety Disorder, and the all-capturing "Adjustment Disorder") if they had to sit down with a researcher using a structured interview and then applying those numbers to interpretation in the Diagnostic and Statistical Manual of Mental Disorders (DSM IV-TR). Again, it seems we find what we are looking for.
So then...I know this was not "little" or brief, but I think that we have to be careful at what type of arguments we offer as proof either for, or against, humans having free will. Like so many things we observe in this universe, free will seems to be quite paradoxical. It straddles the line between sense and nonsense, reason and speculation. The methods of science are susceptible to perversion, just like so many of the world religions became perverted from their original ideas. Any idea or concept can be taken to an unreasonable end when our own passions become intertwined with the quest for knowledge and meaning.
Religion and science do seek the same things (knowledge and meaning) and both have something to say about free will. Like so many of the paradoxical aspects of existence, perhaps both are a little bit right and a little bit wrong. Maybe free will is illusion, maybe it is the ultimate underlying drive to human action. In either case, we seem to think we have it, even if at times we try to convince ourselves that we do not.
"He was a dreamer, a thinker, a speculative philosopher... or, as his wife would have it, an idiot." -Douglas Adams