The Invisible Pink Unicorn (IPU) has been the subject of much debate in alt.atheism. The existence of the creature was first posited in answer to a Christian testimony that God exists. The argument runs like this:
The response makes clear the difficulty of accepting unsupported testimony as regards the existence of God.
Less prosaically, when one is confronted by Christian testimony as regards the existence of God, that appears to be sincere, the unbeliever is entitled to wonder how one can legitimately differientiate between a Muslim who testifies to the reality of Allah, a Hindu who testifies to the reality of Shiva, or the person who testifies to the existence of the IPU, and so on.
Quite frequently those who testify as to the existence of the Christian God based on their personal experience, will note that since they "met" God, there has been a considerable difference in their life, usually to do with some kind of moral regeration. However, one can find examples from any religion where similar claims are made. Again, how are we to differentiate between these claims? Are we to regard the moral regeneration of the one who, for example, discovers Allah and Islam as in some sense unreal simply because our morally regenerated Christian says so?
The long and the short of it is that personal testimony tells the unbeliever nothing except what the believer believes. It is however worth noting that "testimony" plays a large part in many religious movements and serves as a reinforcement mechanism that confirms the believer in their belief.