“I'm not crazy about them.”
You are out shopping with your girlfriend. She asks you what you think of a pair of shoes that she likes. You say this because you don't like them very much.
I'm not crazy about them.
When you say you're "crazy about" something, it means that you really, really like it:
I'm absolutely crazy about this band!
When you say that you're "not crazy about" something, it means that you don't like it very much. This is an example of understatement. You actually don't like something. But instead of directly saying how much you dislike it, you only say that you're "not crazy about" it.
The phrase "crazy about ___" is mostly used by people who are in their mid-30's or younger, although it doesn't sound bad for older people. It's used in casual conversation.