I'd say that the illusion of free will is more important to the existance and function of mankind than the actuality of free will. While determinism may or may not imply lack of free will, this is no excuse to act as if there is no such thing as free will. Apart from the obvious dangers of being wrong - becoming complacent in something because you "have no choice", when you actually do, why should we act differently if we do or do not have free will?
My view on this is probably heavily influenced by behaviorism. If it appears that we posess free will - and it does - then we may as well have it. Our perspective on things would not change if life was predetermined, free will not in existance. We only live once, make every choice once. Not knowing that each choice was predestined to occur in the way it did, every choice appears as if it is freely determined. Why is this any different from it actually being free?
Of course, there is one other issue to mention. Our current scientific understanding of the universe indicates that it is not deterministic. On the sub-atomic scale, quantum theory (which I will admit I only have a vague understanding of) says that there are truly random occurrences. Could the true nondeterministic events occurring in our brains give us free will?
However, I fail to see how true randomness implies free will any more than pure determinism. If you toss a coin, are you making a free choice? There's no reason why a random event should imbue us with free will. However, it does suggest that nothing is predetermined - randomness means that the outcome of events can not be known until they occur, even by some supreme being who set the universe to its initial conditions - but I'd rather not get God into this, as I'm not religious.
To finish, I'll mention that my gut feeling is that I have free will. So I will rationalise things towards that outcome. But do I have a choice, and does it matter?