When describing something as "half decent," it typically means that it is acceptable but not exceptional. There are many synonyms that could be used to replace this phrase, such as average, moderate, satisfactory, or adequate. These words all imply that something is not terrible, but also not outstanding. Another option might be to use the phrase "middling" or to describe something as being "fair-to-middling." This indicates that it is neither great nor terrible, but somewhere in between. Ultimately, the choice of synonym will depend on the context in which it is being used and the specific connotations that one wishes to convey.