Icons are mnemonics, not signposts. Once you know what a button does, an icon can be a great way to quickly recognize it again.
On their own, though, they're terrible. They invite confusion, even after a user has learned what they mean.
An icon in conjunction with text is a wonderful way to help users find things quickly. If I have to choose to design and interface with only an icon or only text, though, I'll go with text.