It's definitely worth it, depending on your tax bracket, you could basically be saving up to 30% of your first $5K of daycare. It does spread that expense out over the course of the year, instead of all at once, but if you got little ones in full time daycare, you'll probably be done claiming by June or July, and just be able to get reimbursed for the rest of the year. A little bit of paperwork, and the rest is a nice bonus!
So here's how it works, say you make $55K and your tax rate is 25%, you need to earn $6,250 to bring home $5K. If you're using your Flex Spending, you only need to earn $5K to pay your daycare $5K, make sense? Plus, it lowers your taxable income, because it comes out of your check pre-tax, by that $5K so that instead of paying income taxes at 25% on $55K, which is $13,750, you would be paying income taxes of 25% (assuming that you are more than $5K over your rate bucket) on $50K, which is $12,500. So, you've saved $1,250 by taking that money out pre-tax and using it, and another $1,250 by lowering your taxable income. Not too shabby!