Reed, a 26-year-old SEO analyst based in central Pennsylvania, considered unlimited paid time off to be more important than salary when she was recently looking for a new job.

And she’s not alone: 50% of workers in the U.S. would prefer access to unlimited PTO to earning a higher salary, according to a new survey of 2,000 workers by the Harris Poll for Fortune. It’s a benefit only about 9% of workers currently enjoy.

While it sounds great on paper, some have found unlimited PTO to be a double-edged sword. Depending on the company, it can either be a godsend for workers who need more flexibility or an easy way for management to cut costs. Without a clear policy and good corporate culture, it can make it harder for employees to actually take time off. 

Unlimited PTO works well “in [an office] culture where you really have permission to take that unlimited time,” says Julie Stone, managing director of health and benefits North America at Willis Towers Watson. “It’s almost the difference between the construct and the concept — which sounds great — and the reality. I’ve seen some places where it’s worked well, but mostly not yet.” 

The other criticism often leveled at unlimited PTO is that workers no longer accrue time off, so companies don’t have to pay out unused time off to workers who decide to leave. 

When making the decision to take a job that has unlimited PTO, Helena Berry, a 30-year-old global operations and senior human resources information systems manager, has some advice: “When you’re negotiating your pay, make sure your base is truly what you want it to be, so then you’re not focused on a payout.”

Berry sees unlimited PTO as more of a psychological benefit than anything else — one she enjoys at her current company. “Unlimited,” she says, is a bait word. “You know it’s not unlimited, but you also know that you’re not restricted.”

She compares the growing popularity of the unlimited PTO to the time when cell service providers rolled out unlimited plans and suddenly you didn’t have to worry about using too many minutes and running up a huge bill. “I just feel less constrained,” she says. “I feel a little bit more empowered.”

To actually enjoy the promised flexibility of unlimited PTO, Berry made sure to apply to companies that she viewed as being more progressive. “I chose companies that really believed in work-life balance, where it wasn’t just a mantra on the wall.”

She remembers the stress of requesting time off at previous companies where her vacation time accrued, and where she felt she had to justify taking any day off to her managers. “They made it seem like you were betraying them for taking time for yourself,” she says. Due to those restrictions, Berry says she “missed out on a lot” — including a best friend’s wedding and some family reunions.

Finding a company that takes its benefits seriously was also essential for Cinneah El-Amin, a 27-year-old technical product manager, when she was job hunting last year. She loves travel and saw unlimited PTO as an essential benefit. “I was very transparent in my interview process that I had some pre-planned travel coming up.” She ended up at a company that touts “tracking-free” PTO, and she put in her first request less than a month into the job.

Still, El-Amin acknowledges that navigating time away is a juggling act. “I really want to use this benefit, but I really don’t want to burn bridges on my team,” she says.

This kind of benefit is only as good as your boss, she argues: “Say I worked for a manager who was not super supportive of me taking time off frequently. That would make it difficult for me to really use the benefit even though it exists at a company-wide level.” 

In that sort of environment, unlimited PTO policies can magnify already harmful workplace behaviors. 

That’s been the case for O., an associate attorney at a New Jersey-based law firm, who sees her company’s flexible PTO policy as basically meaningless due to its workplace culture (she asked to remain anonymous to protect her identity). Her productivity is tracked, she says, and as a result her worth as a worker is “whittled down to [her] billable hours” — a timekeeping practice common in law and consulting.

Her firm’s flexible time off policy, rather than give her peace of mind, leaves O worrying whether she actually deserves to take off. She’s also concerned that as a younger member of the team, she needs to “earn her keep.” With a vague policy in place, she has to rely on anecdotal evidence from colleagues who have been at the firm longer to know how to navigate the policy. 

O. has only worked at her firm for six months. So far, “I just do what I’m told, to a T,” she says.

Ishmael Bishop, a digital fundraising manager at a D.C.-based nonprofit, had a different problem with unlimited PTO. In his early 20s, he was underpaid, working for a now-defunct digital strategy firm, residing in a city with a high cost of living. “Taking vacation was not top of mind,” he says.

Bishop no longer has access to unlimited PTO in his current role, and though he had trouble taking advantage of it in the past, he recognizes it can be “a good company policy,” provided the employers that offer it can commit to the positive intention behind it, encouraging workers to actually take time off.

Note-taking app Evernote takes it a step further — in addition to offering unlimited PTO, the company gives workers a stipend when they take off five consecutive days.

“We lead by example,” says Susan Stick, Evernote’s general counsel and senior vice president of people. Whenever she takes time off, she says she tries to model behavior to make it easier for her team to take time off themselves. The company’s CEO is also “intentionally vocal” whenever he takes vacation as a way of encouraging employees who might be feel reluctant about taking time off.

There’s no executive guidance at O.’s firm. “If I don’t take a single day off from January 1 to December 31, no one’s going to notice, no one’s going to ask, no one’s going to care,” she says. “No one’s going to press me to [take time off].”

She imagines that a more traditional PTO policy would eliminate some of her stress. If she worked for a company where there was a set number of days off per year that she needed to take, there would be a built-in justification to use PTO. At her firm, O. gets in her head trying to reason for any time off: “Every request feels like a big deal. Especially when you’re trying to stay in everybody’s good graces.”

The traditional system is not without its own inherent mental calculus, though. With only a set number of days off per year, every decision to step away from work can feel momentous. It’s why Reed wants unlimited PTO in the first place: “I don’t know how many days I’m going to be sick,” she says.

Earlier in her career, Stick worked under more traditional PTO policies. She says that the question of whether to take a day off to see a baseball game with her kids became a thought exercise because she “didn’t want to burn a vacation day” she might need later. 

Bishop sees unlimited PTO as offering a path forward toward a more gracious approach to workplace productivity in the U.S., where workers feel empowered and vacation is not “earned” but simply a given.

“If you don’t take that time for yourself and rest, you are going to burn out,” he says. “Things that seem easy to you are suddenly going to become harder. Your personal life is going to suffer. Your physical and mental health is going to suffer.”