I've always been fascinated by these types of discussions because they really cut to the chase, that is to say what really is the true essence of consciousness, awareness, thought and all of that. I think I came across the Sam Harris video about a year ago. At that point in time I was already immersed in mindfulness and was well aware of the fact that the mind had the ability of self-reflection. That means we can become the watcher of our thoughts. Even if we can't control our flow of thoughts, we can view them as a non-judgemental outside observer and decide whether or not we choose to attach any importance to them. In my mind that means we have free will to one degree or another.
What Sam Harris is postulating is that even with the ability of self-reflection our choices are still a byproduct of the essence of who we are, much of which we had no control over ergo free will is merely an illusion. Think of your brain as a hard drive that stores away everything you've learned, experienced, come in contact with, all of it, the good, the bad and the ugly. That's your data set, your basic program from which all of your thoughts, choices etc. etc. will flow. Add to that the biological components, genes, mental illness, etc. much of which you had absolutely no control over and it does make you wonder just how "Free" free will really is. I'm not drawing any real conclusions here just kicking the topic around. I can rock and roll with both sides of the debate.