I know it's a completely arbitrary number, but today is the first day we are half-millionaires. :)
By way of context:
I'm 39, DW is 36, DD is 1. My salary is $75K, DW is currently SAHM but will be earning $40K for 9 months when she goes back to work in August 2017. Our annual budget is a bit under $40K, so I've been able to fully fund both my 457 and my 403b even with the loss of her income. Once she's back at work, I hope we can fully fund both for both, but childcare / other child expenses might make that tricky. We should be close, though.
I spent my 20s living frugally but teaching overseas (earning little, fully funding my Roth IRA every year, not saving much beyond that). DW came into our relationship (~8 years ago) living paycheck to paycheck, w/ ~$40K in student loans (now $13K @ 2%, we paid off the 6% ones). She's totally on board, though.
And I'm not including home equity, cars, or anything like that in the $500K number.
Our target is $1.2 million. It's possible (perhaps even likely), though, that once DD is a bit older (maybe 5?) we'll push back ER so that I can switch to a $40K, 9 month position like DW and spend summers travelling.