First of all, let's get a few things straight.
Stalin's rule came at a time when the USSR was isolated from much of the world. The failed German, Italian, and Hungarian revolutions/rebellions had led to a situation wherein the USSR was effectively reduced in scope to the new, post-WWI borders drawn up around the Curzon Line along the Soviet-Polish border. The USSR was meant to be a unified federation of socialist states, Lenin having wanted to stretch it's boundaries all the way to Germany.
Paraphrased from
Stalin: A Biography by Robert Service:
Lenin wanted a general federal union with Germany, with the economy to be administered 'from a single organ.'
Stalin disagreed that Germany would willingly enter such a federation expecting the same rights as the Ukraine, which had been recently absorbed into the expanding Soviet order.
paraphrased From pages 170-80
Stalin's "socialism-in-one-country" policy was an arguable necessity in a world which had become increasingly hostile to the nascent Soviet Union, even more so with the rise of fascism in both Italy and Germany, the consequence of their failed revolutions.
Stalin effectively rebuilt a war-torn nation (or rather, a union of nations) up into an industrial powerhouse capable of holding it's own against Germany and it's European allies.
That alone should be an indicator that Stalin had achieved much in a very short time throughout the 1930's due to the implementation of the Five-year Plans.
What were the personal merits of comrade Stalin?
Besides the fact that he was a powerful leader?
He was furthermore human, and in being human was capable of mistakes and not to mention complex emotions.
Stalin started his revolutionary career in Georgia, his home growing up. He, like Trotsky and others, voluntarily joined the Russian Social-Democratic Labor Party (which would later split into a "Bolshevik" and a "Menshevik" faction). People in the Russian Empire were attracted to Marxism because they saw the oppression going on around them in the towns and in the villages, which it can be said, was being caused by capitalism which operated in tandem with the towering bureaucratic Tsarist state. Lenin and many others whom followed him during the October Revolution (and even before) were very much against bureaucracy. Lenin, in his dying days, would warn against the bureaucratic trend emerging the new Soviet order; He, as with many others whom joined the Bolsheviks, had fought for soviet power-popular, grassroots democracy-and loathed the new bureaucracy which first came into being around 1918, one year into the October Revolution, the Soviet republic having survived only slightly longer then the fabled Paris Commune, an ideal state that Lenin would urge the creation of in his numerous political books and articles.
Stalin came into a position of leadership after Lenin's untimely death. He was a part of the generation which had gone through the arduous July Days and then into the uncharted waters which resulted from the October Revolution.
As such he made many mistakes in both theory and in practice, but that is to be expected of any politician and/or leader.
Successes and achievements?
Bringing the Soviet Union onto equal footing with other powers of his day, chiefly Germany and later the United States.
The Five-year Plans came alongside the tumultuous drive for collectivization of agriculture. Both, despite their costs, made the USSR economically self-sufficient for the most part.
Personal faults?
Years ago I would have said that he was evil, in the same vein as Hitler.
But if one actually closely views Nazi and Soviet society, one will notice a much higher degree of repression in Nazi Germany then in the Soviet Union. Eastern Europe in the '50's during Stalin's leadership was being laid to waste by the Nazis in the '30's and '40's, whom only wanted to exploit their new eastern territories. Education, culture, living standards, etc were to be kept artificially low, as low as possible in fact.
In contrast, Eastern Europe during the Soviet era actually managed to scrape by pretty well despite everything. It can be said that the Red Army's entry into occupied Eastern Europe spared an entire region of mainland Europe from utter devastation.
Another region of the world spared the horrors of fascism was in China. Many in China feared a "change of sky" (or a successful counterrevolution) would come to northern China. The communists there prevailed against all odds both against Japanese attempts at direct colonization and at a no less brutal attempt by the Nationalists to seize control of China post-WWII.
But that's for another thread...
Failures and mistakes?
Tons, but overall he made many successes as well. No one, esp. someone in his position, is immune from making mistakes.
Whoever came up with this idea first (debatable in its own regard) is completely and utterly irrelevant here. 
- By FiveofSwords
- By Unthinking Majority
- By Potemkin
- By Deutschmania