The phrase "am useful" implies that someone or something has value, purpose, or usefulness. The opposite of "am useful" could be "am worthless," which means that someone or something has no value or benefit. Alternatively, "am harmful" could be an antonym for "am useful," indicating that someone or something causes injury, damage, or harm. Other antonyms for "am useful" could include "am irrelevant," "am unnecessary," or "am impractical." Depending on the context in which the phrase is used, there may be different antonyms that adequately describe the opposite of "am useful.