Hi Penny - I implemented a FTO policy at my prior company.  We were small (less than 25 people) and project based, focus was on delivery and not hours worked.  The move was two fold to eliminate vacation carryover balances from the financials and to stop the admin work to track and manage time off accruals and usage.  Employees loved the change, made them feel like they were treated as adults who can manage their own time.  I found I had to push people more to take time off with the change and for guidance for new workers we said up to 3 weeks a year is reasonable and if you need more please talk to us.  Approval was needed for any time over 1 week to be sure appropriate coverage was in place.   

At my current company, we have unlimited flex time with +3,000 people.  I do not manage the overall program, however personally I find the same struggle of not taking time or thinking about it.  

Depending on the work you do and the culture, FTO may be a huge morale boost and from my experience I had the challenge of getting people to take time.  

Hope this helps and let me know if you'd like more details.