Returning rental car and flying out of DFW, what are my options?

I've never been to DFW. I'm renting a car one-way, driving it to the Alamo at DFW and returning it prior to a 6 AM flight. I heard there is a direct shuttle from car rental to the terminals. Does it run 24/7? Is it reliable? How long will the shuttle take (including wait time)? Are there better options than the shuttle?