Free Will and Suffering
Christians typically use the free-will argument to address the problem of suffering: Suffering exists because we humans have the free-will to sin and cause suffering; God gave us free-will so we wouldn't be mere robots, and can freely choose to follow or disobey God; It had to be one or the other, free-will or robots.
This argument is a decent attempt to shield God from being responsible for suffering, but most people, including several Christian philosophers I've read, acknowledge that it isn't a slam dunk. The problem of suffering remains a theological and philosophical problem.
The main problem is that the free-will argument includes a rule: there can either be free-will or robotic humans. If God had to operate within this rule then he is not ultimately God, because he is subject to some external rule outside himself. If God did not have to operate within this rule, then he could have created infinite alternative rules which do not result in so much needless suffering.
Along the same lines, the apologists cosmological argument states God is the beginning of all things, the first uncaused cause. God existed "before" anything else material or non-material existed, including evil. Then God acted, performing the first cause from which all subsequent causes and effects trace. This first act eventually resulted in, among everything else, a new thing called evil. If God's original act of creation is the ultimate first cause of everything, it is necessarily the ultimate first cause of evil. To conclude that free-will caused evil is to neglect the question of what caused free-will, and specifically our particular brand of free-will.
If God is the first-cause, then everything, including evil, must causally trace back to God.
Free Will and Salvation
Christianity claims that the paramount decision facing all humans is whether or not to follow Jesus, upon which our salvation depends, and we have free-will to make that decision. The problem with this claim is that it's unclear whether Jesus really exists, which makes it unclear whether "following" him is really the right thing to do. So the decision to follow Jesus is actually a secondary issue, wholly contingent upon a guess as to whether Jesus exists. This could be forgiven were Jesus' existence largely the more rational guess, but most of the world finds the opposite is the case. Most of the world is sincerely guessing that Jesus doesn't exist, not because they will to do what is wrong, but because they will to do what is right!
Because of this, I find that "free-guess" is more applicable to the Christian view of salvation than "free-will". Let me elaborate with an analogy.
It would be like God placed all humans in a grove in the middle of an enormous forest. To exit the forest, there are a thousand paths radiating outward from the grove, each traversing different terrain, each presenting different challenges and rewards, some of which intersect other paths at various points. Each path takes a lifetime to traverse. At the end of each path is a unique exit door from the forest, which we must enter upon arrival. Nobody can see what's on the other side of the door until they themselves enter. The catch is that one of these doors leads to eternal Paradise, all the others lead to eternal Prison, and God has not clearly marked the paths.
Rather, the humans instinctively congregate into numerous feuding factions with competing ideas about which path leads to the door of Paradise, many claiming to have received this information from God. Now, it would be silly to say that God has given them free-will to choose Paradise or Prison, because he's concealed these outcomes behind a barrier of mystery and unknowing. Rather than choosing between Paradise and Prison, God has left them to guess which path goes were. If a human cries out, "Lord, I choose Paradise, no matter the sacrafice!" God responds (if not verbally, then through silence), "That's not how it works, now guess a path." and leaves him/her subject to the pursuasive power of the numerous factions. Not suprisingly, the majority of the humans end up picking the wrong path, mostly by an unlucky guess.
When an unlucky souls open a door to eternal Prison, is God to say, "This was your choice, you should have chosen Paradise. Now face the consequences of your decision"? That doesn't make any sense. The vast majority willed to choose Paradise with all their hearts, they simply didn't know which path to take and were forced to guess.
In order to have free-will rather than free-guessing in the forest analogy, the paths must be marked. Not by other fallible humans on the same journey, but by God, who alone has knowledge of where each trail ultimately leads. We must understand the decision in front of us and choose our fate accordingly. Knowing then which path leads to Paradise, we can choose to either follow that path no matter how treacherous and painful the journey, or we can choose take an alternative path because it looks temporarily easier or self-gratifying. That would be true free-will.
Despite the countless man-made signs screaming out "This Way! God Told Me!", God has not marked the path. Salvation, if there be such a thing, wouldn't plausibly depend on an ill-informed guess. The concept of free-will doesn't add much rationality to the Christian view of salvation, because the Christian view of salvation depends ultimately on a free-guess.
Free Will and the Bible
Free will is used to defend the Bible-based theory that there exists a good, omniscient, omnipotent god. In order to explain the apparent lack of this god's intervention in the world today (especially with regard to suffering), Free Will is argued. However, the Bible is a collection of stories which depict God heavily intervening in the world, thereby interfering with human Free Will. So Free Will, while used to support the Bible's theory that god exists, doesn't have much ground in the Bible's depiction of that god.
Consider just about any Bible story, such as Jonah and the whale, Noah's Ark, Tower of Babel, the parting of the Red Sea, the walls of Jericho, the prophets, the Ten Commandments, Jesus' miracles, etc. They all depict an all-powerful god intervening in our world, obstructing human free-will in some manner.
Today when Christians attempt to explain horrible realities such as Sierra Lamar's tragic abduction, they'll often say "Well, God gave her abductor Free Will." There are currently thousands of people praying that Sierra Lamar turns up alive. If, tragically, the police's suspicions prove correct and she has been murdered, many Christians will probably use similar Free Will arguments to help explain why God didn't answer their prayers, intervening and saving the young girl from her captor.
But again the Bible is full of stories of God doing exactly so: freeing the Israelites from Egypt, giving Sampson supernatural powers over his enemies, freeing Peter from prison, handing over numerous enemies in battle, promising that enemies will be crushed.
It seems unreasonable to use Free Will to defend the apparent lack of intervention of the god depicted in the Bible, when that god is depicted as intervening over and over again in dramatic visible ways.