A necessity for many who travel to the Philippines via Manila Ninoy Aquino International Airport, Manila is good for partying and, if you have a spare day waiting for flights, the Fort and Intramuros area is worth checking out.
But, bluntly, considering the traffic which is up there with the worst anywhere in the world – dirtiness and general lack of any real highlights, I would sack it off and spend your time enjoying some of the other wonderful experiences to be had in the rest of the Philippines. In particular, one of the best travel experiences on the planet Island hopping Coron to El Nido with Tao Philippines, and one of the best scuba dive locations in the world at Tubbataha Reef.
Or, if looking for a big developing South East Asian city to explore, put the likes of Bangkok, Ho Chi Minh City, Hanoi, Yangon, Phnon Penh ahead of Manila.