I've been using one for years in NY. Starting Jan 1 they take out a pre-determained amount of $ from each pay check (before taxes). Then every time I (or my DH/DD) have a copay for doctor's visit, prescriptions, etc I either use the "debit card" that they provide, or save the receipt and submit it to the flex account company. If you use the debit card (not all pharmacies/stores/doctor's offices takes them) you have to save the receipts. They can audit you. I have been audited once.
If you pay out of pocket, then submit the receipt and they will either send you a check or deposit into your checking account (if you have that set up). Our Flex Company takes FOREVER to reimburse. So I like to use the debit card when I can.
I have used the account to pay for eyeglasses (Rx) & contacts, medical supplies that I had a Rx for, and even orthodontist costs. They use to let you use it for over the counter medicines like motrin, but they changed the rules.
It is definitely worth doing. You just need to have a good ballpark of what you spend each year. Like a PP said, underestimate. If you don't use it by the end of the year, you loose it. I always buy eyeglasses in December.
A few years ago, I bought expensive Rx eyeglasses in January. Only a few dollars had been taken out of my check. I submitted the receipt right away, and got the full amount within 1 month. I then had a negative balance until my paycheck deductions paid it back.