I've ridden the K2 Formulas and Union Force
I personally HATED union bindings. Love the construction though, feel totally bomb proof. Toe cap didn't fit, I had mad pressure points - Toe cap was either too big, too small. Not for me. (though in the shop they looked like a perfect fit)
K2 - Is pretty much my go to brand now for bindings, I've just had great experiences with them. Had the 2012 Formulas and now got the 2013. Great all mountain binding at not a bad price either.