Southwest is great for the market it serves. But I typically search both Southwest and kayak.com, where I sometimes find better fares for even markets which Southwest
does serve. Then I go to the individual airline site unless Kayak is pointing to best airfares with multiple airlines. Then, I use cheaptickets, priceline or whomever Kayak is pointing toward.
Keep in mind your best fares will usually require mid-week departure and a Saturday night stayover.