I did, before they pulled out of Fort Collins/Loveland CO. They weren't bad, but you've got to watch all the hidden charges. Like you said, $5 just to print your boarding pass.
There were a few times where the total cost to fly with G4, including bags, seat assignment, priority boarding, print your boarding pass, etc. was less than driving to Denver to catch Southwest. Also factor in that Southwest has a FF program, so discount your SWA flight 6/70 = 8.57% for that. Even with all that factored in, G4 was a bit cheaper and I saved the 70 mile drive to DEN.
However, what G4 can't give you at any price is that they don't have several flights a day going from your city to one of their hub cities. So, when a plane is delayed for whatever reason, you are stuck in your departure city, whereas SWA has another plane going in a couple of hours and several other options to get you to where you want to be -- perhaps delayed but that is better than "not at all today."
G4 isn't bad ... just calculate carefully. However, since they left KFNL, they really are not an option for me anymore. If you are based in LAS or other G4 hub city, they offer some interesting alternatives in smaller airports -- if that smaller airport is close to where you need to be.