The Soviet's New Economic Plan was indeed a successful plan in establishing a comparatively strong postwar economy for the Soviet Union, but there were many imperfections: food prices fluctuated tremendously, the gap between the wealth of people increased. Was it a successful plan of Leinin?
A recommended reading here is Farm to Factory: A Reinterpretation of the Soviet Industrial Revolution by British economic historian Robert Allen. I don't know the literature in detail, but Allen is generally very well respected for his work. (Google finds a review of the book here). From Amazon's page on the book:
For more details: read the book! :-) (if you are on a university network you might find a paper on the same subject by the same author here)
No. It is generally considered to be a failure.
The chief problem was the recurrent scissor's crises due to the agricultural sector not being incorporated into a commodity economy. As the price of agricultural goods declined, peasants would withdraw from the market economy and the (very few) Kulaks did likewise, reverting from small rural capitalists to rich peasants. When the market economy did not operate purely in the interests of the peasantry, they retreated from the market, creating imbalances in the agricultural to industrial price balance.
See the Nove Millar debate: Millar, James R. and Alec Nove. “A Debate on Collectivization: Was Stalin Really Necessary?” Problems of Communism 25 (July/August 1976): 49-62.
Normatively, if we consider the purpose of the Bolshevik party to usher in a classless society through the agency of the working class—and if we consider this to be an "economic" goal—it was a god awful disaster and the worst case of ratting out the class to the boss by non-workers in a so-called worker's party ever seen. Simon Pirani's work demonstrates that Lenin dismantled the workers state in 1921, replacing it with a Party state. Any of the sociology of the NEP will show that institutions such as one man management dismantled significantly much of the property control that workers had over plants. So the NEP smashed the institutions of the workers' state by dismantling the political power work place committees held, and it significantly dismantled the economic ownership of the workplaces by works committees.
Finally, by the late 1920s "ultralefts" in the lower orders of the Bolshevik party were demanding collectivisation, and many urban workplaces were demanding a campaign of terror against the countryside due to the long term lack of adequate food supply to the cities. If the NEP was meant to be a long term and positive economic process, these political problems couldn't have arisen from below. (See Conquest or Solzhenitsyn)