Southwest Airlines gives you the cheapest fares, but at what price? I've flown with Southwest twice, and it was okay, but as they say, this is a no-frills airline.
I found that the fares were the best by around $100, but I also know now that flying with Southwest means you get a whole bunch of like-minded other budget travelers, and sometimes being stuck on full flights with loud families with babies and as much carry-on luggage that they can pile in is not how I like to fly.
I can't fault Southwest - you do get good deals. Great deals. If you need to get from A to B as fast as possible at the lowest price possible, go with this airline.
However, you do get to realize that everybody is thinking like you are: price only. Your only savior is trying to sleep amidst the din of people.
I suppose I was only not impressed with the clientele, because the airline itself is fantastic for a bydget flier. They get you there and the planes are clean. Southwest only uses 737s, I've heard to keep maintenance costs down (a fleet using only one plane means lots of spare parts to work with). However, these planes are usually filled with people at max capacity for profit purposes, and the amenities are usually sparse (again, no-frills airline).
If you have a short hop to make, fly this. If going cross-country, then you're in for a subpar flying experience depending on who's around you.