These are stupid, and I wish they'd die, for multiple reasons.
First, the list of things that are allowed and not allowed is completely arbitrary and random. Feminine hygiene products are allowed, but baby formula isn't. Wtf why? And now someone's gotta maintain this list of items and enforce it (Wageworks et al), and process claims made manually. So now we've just added another actor in the whole system that can add their own profit margin on top. All so we can save a few bucks in paying taxes.
Second, these aren't available to everyone (just like 401k plans, the other symbol of taxation inequality). It's a benefit that has to be offered by your employer, and there are plenty of shitty employers out there. The people who could most use a tax break on a $10 bottle of aspirin are the least likely to get it.
Third, the use-it-or-lose-it nature of FSA money. And if you lose it, the money goes back to the employer. Again, wtf why? You're basically placing a wager against your expected healthcare expenses every year at open enrollment time. For really no clear benefit. Why can't the "debit card" work like a "credit card" instead, with payments made directly from your paycheck? There's no guessing involved, and no one ever over- or under-contributes.