I have used them numerous times. They are on site at the airport. I never had any problems with them. And when I've used them they have never had a line.
Used L&M, was very skeptical at first cause their rates were so low but they were GREAT! Picking up the car at the airport was an especially nice bonus as we were traveling with two small children. We were off the plane, luggage in tow and in our rental car 22 minutes after we landed...
Be careful though, they appear to charge quite a bit more for taxes than National and others do. Make sure you compare the total price to make sure it is a good deal.
We have used them two or three times and have gotten great service and brand new cars every time! I was worried because I've heard they push the insurance, but there was no pressure any of the times I've used them (and no line).
Barb