Frequent Travel section contributor Chris Elliott has done a Travel Troubleshooter column on Google Flight Search, a new competitor in the busy realm of searching for airline service, that needs to be read carefully. After a slow start, keep reading and you'll see some flaws in the Google system that Elliott finds.
In addition to what he notes, be aware that Google Flight Search doesn't list fares or flight details for low-cost airlines, including Southwest, JetBlue, Allegiant and Spirit, that aren't full participants in the big airline reservation systems. What you see with Google are only the schedules and fares for the older major airlines and links to their Web sites. If you want to get a more accurate view of all that is available, you also still need to go to the low-cost carriers' Web sites. Also excluded are online travel agencies or meta-search engines including Expedia, Kayak, Orbitz and Travelocity.
My own search of philly.com says the column hasn't been printed yet in the Inquirer but is available online, and you can read it at this link.