A signed IOU is a contract. IOUs are used in court as evidence. Whether it "stands up" depends on what the document contains, whether the terms and conditions in it are legal, clear and complete, whether the signature can be determined to be valid, and so forth. Other things may also come into play, such as whether the person that signed the IOU was authorized to do so (whether they were under age or incapacitated, coerced, etc.)