Alan Furst described it as a kind of romantic improvisation in impossible circumstances. Foreign Policy says it's another phrase for the black market, and that it may be the future. The article cites sources claiming:

"... half the workers of the world -- close to 1.8 billion people -- [are] working in System D: off the books, in jobs that were neither registered nor regulated, getting paid in cash, and, most often, avoiding income taxes."

"... people in the European countries with the largest portions of their economies that were unlicensed and unregulated -- in other words, citizens of the countries with the most robust System D -- fared better in the economic meltdown of 2008 than folks living in centrally planned and tightly regulated nations."