The New Deal was clearly successful in preventing the failure of democracy and the rise of a state-planned economy in the United States. Even critics of the New Deal would have to acknowledge this point. Those who criticize the New Deal tend to do so on two primary grounds: it failed in its major objective of ending the Great Depression, and it made too many concessions to the left.
The second criticism is relevant to this question. One might see Roosevelt's policy as being analogous to an inoculation. By injecting a small amount of Socialism into American society, modifying the Social Security system and creating a large state-sponsored program of public works, the president preserved American capitalism and democracy.
Complaints that the New Deal increased bureaucracy and the national debt are essentially an assertion that the price Roosevelt paid for stability was too high. However, these are countered by liberal criticisms that the New Deal left the structure of American society, specifically the wealth gap, fundamentally untouched. The balance of these criticisms from the political right and left is a testament to Roosevelt's achievement, and the fact remains that the New Deal did save capitalism and democracy in America, whether or not this end could have been achieved some other way.