Every time I've strayed from the "3 majors" when renting in the US, I've encountered a big hassle (and often a long queue for the rental counter), at a minimum. For me, that alone is enough to avoid the second-tier and off-brand companies. Given the plethora of anectodal evidence of scamming (outrageous charges for "damage" - I wonder if THEY pre-scratch the cars and then charge every subsequent sucker who they can get to pay for the SAME damage - insurance ripoffs, deceptive/improper fuel charges, etc.)

Bottom line - stick with Hertz, Avis or National and it's much less likely you'll end up hassled and/or ripped off. I personally use Hertz exclusively, and if Hertz isn't an option I'll use Avis. With #1 Gold (express rental) I am usually in my vehicle, often with a free upgrade, driving away from the airport within 5 minutes of arriving at the Hertz location and have never been stuck with ripoff charges. The one or two times I've been overcharged, one quick call to Hertz saw the overcharges removed. One time they (wrongly) accused me of damaging a car in KOA, and again, a quick call and a one page fax to Hertz and no further hassles.

I'll gladly pay $5-10 a day more for this track record; often, I don't pay any premium at all.