Many people swear by Southwest, it's the carrier they always use, it's the carrier they brag about to all of their friends. Some might even say that I'm that way about American Airlines. I'd argue that American Airlines isn't the best carrier out there but their air miles are easily worth the most, which is why they are the best choice for loyalty for now.
Southwest has a few problems going for it at the moment. Their network is heavily focused on the domestic market. If you never travel internationally, then Southwest may be a good choice for you. Myself, international travel is very important, and when I fly internationally, I want to do so in a premium class for the sake of comfort. Flying with Southwest builds air miles with a company that doesn't offer a premium service, and will not benefit me at all for the long trips where great seats make the big difference.
There is another issue at hand that turned me off to them long ago. A few years back, Southwest was accused and eventually fined for using bribes to avoid mandatory safety checks of their aircraft. These checks are required under FAA regulations, they are very extensive in some cases needing the planes to be pulled apart to check for problems. While personally I have worked under these regulations, and think that in some cases these regulations go too far, but they are there for a certain reason.
When an aircraft gets old, many problems come into effect from repeated takeoffs, landing, and pressurizations of the cabin. These cycles can cause cracks and fractures to start in the metal. The FAA requires these checks in order to assure that the aircraft is safe, and if there are any problems found, that these problems are addressed quickly.
Southwest was found to have bribes FAA officials so that they did not have to preform these checks. Checks on the aircraft can cost upwards of a million dollars for the very extensive tests, and takes the aircraft out of commission for long periods of time. Keeping these planes in the air saves the airline millions of dollars. This comes at a hefty safety cost to the pblic and employees flying these aircraft.
Personally, I take safety issues very seriously and the fact that Southwest is willing to bypass safety regulations to help their bottom line speaks to a problem with the managing of the company. For me, this means I do not fly with them. Maybe I should give them a chance to make it up to me... but like I said before, their flights are uninteresting and their points are mostly useless to me.
If Southwest flys to the places you need to be and you trust that they changed, by all means go ahead and use them. Personally I don't have a compelling reason o fly with them. American Airlines probably isn't perfect either, but they do fly to where I need to be.
Are there any airlines you refuse to fly with?