While the popularity of car sharing has recently grown in the United States, Hertz's 24/7 service has had some bumps in the road.

Hertz Global Holdings has been dealing with a plethora of customer complaints about its car-sharing unit, which was recently renamed Hertz 24/7, Bloomberg reported. The angriest customers have been sounding off on social media including Yelp and Twitter.

Some of the complaints may be the fault of the unknown person who used the car before. In the car-sharing business model, a lot depends on the previous user.

"The customer is part of your service-delivery mechanism," Mark Norman, president of Zipcar, said.

For Hertz, customers have complained that the company failed to make reserved vehicles available at the appointed time and place. Hertz has also allegedly changed a pickup to a location miles away, giving as little as 15 minutes warning when reservations are changed, and employed brusque or clueless telephone operators unable to resolve problems.

"It's a new business, and like all new businesses, it has growing pains and we're trying to learn as we go along," Hertz spokesman Richard Broome said.

The company charges $10 for every 15 minutes a car is kept past its return deadline, but customers pay no penalty for returning vehicles to an alternate location, so long as it isn't "unreasonably distant" from the pickup spot, Bloomberg reported.

The sharing technology has been equipped in Hertz cars in about 1,500 cities worldwide, with the largest U.S. market that of New York City.

The car-share business has grown in recent years among traditional car-rental companies, which have huge fleets of cars that can sit unused at any given time. Companies can equip a car with GPS and other tracking software, park it in a densely populated urban neighborhood or college campus, and have it used nearly every day to boost their finances.

Car-sharing has been common in Europe for decades and caught on in the U.S. with Zipcar, according to The New York Times.

One-way or free-floating car sharing uses GPS and smartphone apps for more service. Cars are parked on city streets, and users pick up cars parked nearest to them. Instead of bringing the car back to a lot, users leave it wherever they find parking near their destination and are charged for the amount of time they spend driving.  

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