http://www.kaiserhealthnews.org/features/insuring-your-health/2013/070913-michelle-andrews-on-cost-sharing-subsidies.aspx
A family of four whose income is between 100 and 150 percent of the federal poverty level ($23,550 to $35,325) will be responsible for paying 6 percent of covered expenses out-of-pocket compared with the 30 percent that a family not getting subsidized coverage would owe in a silver plan. A family with an income between 150 and 200 percent of the poverty level ($35,325 to $47,100) will be responsible for 13 percent of expenses, and one with an income between 200 and 250 percent of the poverty level will be responsible for 27 percent ($47,100 to $58,875).
In addition, people who earn 250 percent of the federal poverty level or less will also have their maximum out-of-pocket spending capped at lower levels than will be the case for others who buy plans on the exchange. In 2014, the out-of-pocket limits for most plans will be $6,350 for an individual and $12,700 for a family. But people who qualify for cost-sharing subsidies will see their maximum out-of-pocket spending capped at $2,250 or $4,500 for single or family coverage, respectively, if their incomes are less than 200 percent of the poverty level, and $5,200 or $10,400 if their incomes are between 200 and 250 percent of poverty.
This helps lower-income families a great deal, but leaves middle income families with the need for cash up front.
Note that cost-sharing reductions are for silver plans.