Whether the effects of World War II on the Soviet Union were positive or negative surely depends on the time frame in which we consider the question. For the short and medium terms, the war’s impact would have to be seen as negative. In the longer run, the war might be said to have had somewhat of a positive effect on the USSR because it helped the country become a world power.
In the short and medium term, WWII was a terrible thing for the Soviet Union. More than any other country, the USSR was devastated by the war. The Soviet Union lost a huge number of people to the war. An estimated 24 million Soviet citizens died during the war. Something like 10 million of these were military deaths while others were killed by things like starvation. This amounts to approximately 14% of the pre-war population of the country. This is a higher percentage than were killed in Germany, even counting the people (like the Jews) killed by the Germans themselves. This horrible human devastation alone would have made the war a bad thing for the country.
But the human devastation was not the only effect of the war. Instead, the war wreaked economic havoc on the country as well. When the Germans invaded the USSR in 1941, Stalin’s troops conducted a scorched earth policy as they retreated. They tried to destroy as much as possible as the pulled back so that the Germans could not find anything that they could use. At the end of the war, as the Germans were retreating through the Soviet lands that they had occupied, they did much the same thing. Thus, many areas of the Soviet Union had crops and factories and other things destroyed not once but twice during the war. This destruction took a long time to rebuild after the war.
However, if we look at things in the longer term, we can argue that the war was good for the Soviet Union. One aspect of this was the fact that it allowed the USSR to gain more territory. After the war, the Soviet Union was able, for example, to retake the Baltic countries which had been taken from it after WWI. More importantly, though, the USSR was able to dominate all of Eastern Europe. Countries like East Germany and Poland became satellite states of the USSR. This added immeasurably to Soviet prestige as the USSR became the only country that could rival the United States for world power. The war also helped the USSR achieve this kind of power because it destroyed all potential rivals for the Soviet Union other than the United States. The USSR could no longer be threatened by Germany or Japan as it had been before WWII.
If we take a long-term view of Soviet history, we can at least argue that WWII had a positive effect on the country. However, the war’s effect was extremely negative in the short and medium term because of the destruction of human and other resources that it caused.