I had an excellent experience with Thrifty! I booked my car about a month in advance through Southwest (they're a Rapid Rewards partner, so you get points for the rental) and got a pretty great deal.
After a very quick check-in process, I was sent upstairs to pick my car. This was the first time I've rented from an agency that doesn't assign a specific vehicle to you. I chose a bright blue Dodge Avenger and was on my way!
At the end of my trip, I was a couple of hours late in returning the car. This wasn't an issue at all! We called Thrifty to see what their policy with late returns is and were told that, as long as we returned the car on the day it was due, we wouldn't be charged. If we had taken the car back the next morning, we would have been charged for another day, which is fair.
Every staff member I interacted with was polite and friendly; the whole process was a breeze!
Tips: Know the gas prices in the area where you will be returning the car; filling up on your own may be cheaper than their gas rate. If your credit card offers rental car insurance, book the rental with that card; that way, you can refuse the rental company's insurance and save $, but still have coverage. Do some research on the cars they offer so that you can pick the most fuel-efficient one.