We can argue about it's long-term sustainability because of the baby boomers retiring and all that, but just in terms of administration and execution, the social security program is incredibly well-run and effective. According to internal reviews, the error rate is amazingly low:

The FY 2011 error rates for RSDI overpayments and underpayments were 0.32 percent and 0.13 percent, respectively

http://www.socialsecurity.gov/improperpayments/documents/AO_Report_2013.pdf

And the administration overhead cost of the program is relatively low as well, considering it's very high accuracy rate. It looks like the overhead and administrative costs of the program has been less then 1% of the total cost of the program every year since 1989.

http://www.ssa.gov/oact/STATS/admin.html