We mostly fly USAirways out of PHL. We are heading to WDW in November on SW because they had a great sale (and USAirways eventually matched those fares). Since SW moved in, it has become a much more competitive market, at least for travel to Florida. Retrieving luggage after you land back in PHL continues to be a hit or miss proposition; sometimes it takes over an hour, sometimes it appears right away. No such issues when traveling with SW.
We took two voluntary bumps back in June from a 6:30 a.m. Orlando flight. Frankly, that surprised me because that flight is rarely full. But it was a pleasant surprise and came in very handy for our flight to Dallas in two weeks. SW does not (cannot) fly directly to Dallas from PHL because of the Wright Amendment, so fares to Dallas remain fairly high, IMHO. Flights on other carriers (American, Delta, etc.) were pricey as well. So, the bumps were fortuitous for us.
I have noticed service trending downward since 9/11 on USAirways, but not to an egregious degree. Whether the merger with American West will be a positive situation remains to be seen. Since they are the dominant carrier in PHL (and seem to be committed to the area), we will continue to use them as well as SW. I am happy SW is here because it does help keep pricing lower. If Jet Blue were to serve here, I would be delirious, but Im not holding my breath waiting for that to happen.