Chile, based on Milton Friedman's ideas. It has been problematic -- all political sides agree that it needs major reform -- but not catastrophic. This has been partly a matter of luck: the funds lack diversity, and some years of high returns have been due to copper price spikes (much like other boom years in the Chilean economy).

I think it's fair to say that if an American worker was faced with the Chilean program, with its large investment fees, that they would be extremely pissed, compared the very small cost of administering the Social Security program.