It totally depends on the situation. My mother never used the money for my benefit. We were often homeless (lived in our care or with friends/family temporarily) and dependent on church food. Her own income and the money my dad sent via the child support system typically went to her drug habit, her hair/make-up/nails, her "clubbing" attire, or (on rare occasion) to gas for her vehicle that was only occasionally used to pick me up from school (I typically rode the bus or my bike, depending on the weather). She even had the balls to tell me once that I couldn't go live w/my dad 'cause they'd allow him to stop sending her child support checks, and she deserved that money for putting up with me.
Conversely, I have a friend who, albeit TERRIBLE at budgeting money, really does apply all of the child support she receives to the care and comfort of her children. Whether it goes to rent, food, school field trip fees, or healthcare costs...she's never used that money on herself, and she wouldn't be able to survive w/o it unless she were forced to maintain a budget w/constant supervision....and even then she'd struggle.
I always thought it was funny that my dad got saddled w/child support AND the obligation to provide health insurance. He also usually ended up being the sole provider of clothing (that didn't come from charity, anyway), school supplies, toys/entertainment (he bought me two bikes while I lived w/my mom because the one I took to Michigan got stolen and she pawned the first replacement) and basic grooming costs (occasional haircuts, toiletries, etc). His required child support was $400 a month, but he probably spent almost $1,000 in total, since he was self employed and it's expensive to insure JUST children, especially when the child lives in a different state than you do.