Some say that a God who changes his mind is ridiculous so that either the bible is wrong or the God of the bible is not worth following. Some even think that this means you can make God do what you want (like giving you material things if you just have enough faith).
Here's a scripture to put this into perspective:
"Remember this and stand firm, recall it to mind, you transgressors, (9) remember the former things of old; for I am God, and there is no other; I am God, and there is none like me, (10) declaring the end from the beginning and from ancient times things not yet done, saying, 'My counsel shall stand, and I will accomplish all my purpose,' (11) calling a bird of prey from the east, the man of my counsel from a far country. I have spoken, and I will bring it to pass; I have purposed, and I will do it.
If you read Genesis 32 carefully, you can see that there was no change of mind in any case. God merely relented from pouring out his wrath on Israel. If that's too weak for you, at least know that God had already made up his mind because his purpose will be accomplished no matter what we want.