What are the advantages and disadvantages of constitutional monarchy?

Constitutional monarchies work so long as the political power still resides in the people, and the monarch doesn't try to usurp power and transform the government back into being an absolute monarchy. In some cases, keeping the monarch in tact can have a stabilizing effect on the country. In Thailand, the king has been able to avert any lasting military coups, and while he has no political authority, he is respected by the people, and has used that power to help maintain freedom for the people.