I was just at Epinions.com and it seems Thrifty is one to stay away from. Many people said their 'low price' was a bait and switch sceme. Seems they have a 'habit' of loosing reservations and then charging a much higher price...$200 more in the case of one family.
I used Alamo last year and didn't have a bit of trouble. Quick in and quick out. I had reserved an econobox and they upgraded me to a compact, no charge. I would go through them again if their rates were cheaper.
All the rental car companies are at Epinions.com...take a look.