I've known about Southwest for many years, but they only started flying in and out of Philly in May 2005. I have friends who live in New Hampshire who use Southwest all the time.
At first it appeared that Southwest was not competitive with the other airlines flying in and out of Philly. I could get better rates to Orlando with United/US Airways. I saw the DING software on their website, but DH and I are both a bit wary about that sort of program. We didn't know what sort of sneaky little things were built into it.
We did use Southwest to book a flight to New Hampshire to visit our friends last October.
One day, however, I saw a message here talking about a great DING rate PHL-MCO. I immediately downloaded the DING software and was able to book that rate for our trip to WDW in May 2006. This was when Southwest was offering DING fares several months in advance. I had already booked the reservation at the regular rate with Southwest, so I canceled my original reservation and rebooked at the DING rate.
The rest, as they say, is history. Now I can't live without my DINGs.