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?
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)
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:
Although the Russian economy began to develop in the late nineteenth century based on wheat exports, modern economic growth proved elusive. But growth was rapid from 1928 to the 1970s--due to successful Five Year Plans. Notwithstanding the horrors of Stalinism, the building of heavy industry accelerated growth during the 1930s and raised living standards, especially for the many peasants who moved to cities. A sudden drop in fertility due to the education of women and their employment outside the home also facilitated growth.
While highlighting the previously underemphasized achievements of Soviet planning, Farm to Factory also shows, through methodical analysis set in fluid prose, that Stalin's worst excesses--such as the bloody collectivization of agriculture--did little to spur growth. Economic development stagnated after 1970, as vital resources were diverted to the military and as a Soviet leadership lacking in original thought pursued wasteful investments.
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)