It depends and is somewhat complex. It is immediately good for most people because that is more money in their pocket for food, bills, and discretionary spending. However, it is bad for oil businesses and the regions that depend on the oil business for jobs and tax income. Both ends of the spectrum can affect the economy.
Texas for example has lost tens of thousands of jobs due to cheaper oil. Those are all people who are no longer paying taxes and who may be receiving government benefits in some form or another. Fortunately, Texas has a pretty diverse economy even within the energy sector (natural gas, solar, wind, fracking, etc) so the state as a whole has not been hit too hard and the void has been filled by other industries growing (IT, import/export, healthcare, defense, agriculture, and entertainment and tourism) and moving here. However, all those people who have lost their job probably don't think cheaper oil is good. Neither does the city of Houston which may be in a recession specifically because of the decline in the price of oil.