I travel frequently for work and rent across all the major rental agencies (Hertz, Avis, National) and their discount subsidiaries (Thrifty, Budget, Alamo). These days the process is super smooth.
If you register with the agency's free rewards program, the process is usually:
1) Bypass counter to the rewards members area 2) Pick up pre-assigned car or choose your own from the designated pickup areas 3) Drive to exit, show drivers ID, and leave.
When returning, most agencies allow you to just drop it off and get your receipt delivered via email.
I've tried flightcar a few times and have no complaints about the experience itself - it just wasn't any better than going with a major agency, with more downsides: 1) weirder off-airport location (makes returning more difficult - vast majority of airports have clear signs for car rental dropoffs that are super easy to follow) 2) not as consistent in terms of car quality (major agencies standardize their lineups across a few models and years) and maintenance - harder to change/swap cars 3) not as good support (stuck somewhere in random CT, hertz locations everywhere, flightcar = BOS)