This is from today's White House Fact Sheet:
Today more than 6.1 million people are covered by HSA plans, up 35 percent since 2007. Low- and moderate-income Americans and those previously uninsured are enrolling in HSA plans. In 2005, the most recently available data, more than one-third of HSA policyholders had incomes under $50,000 per year, and one-third of individual HSA policyholders were previously uninsured.
There are about another 6 million people with Health Reimbursement Arrangements (HRAs). These are completely flexible accounts which employees can use to pay medical expenses not paid by third-party insurance. These are also consumer directed health care accounts.
The drawback is the people with HRAs can never take the money out in cash.