The problem with Socialism is the wrong leader can turn things bad in a hurry. This was the problem with the Soviet Union, and leaders like Stalin who were power driven and mistreated the people eventually led to its demise. If Lenin had made it 10 or 15 more years I honestly think he could have made the Soviet Union one of the most powerful and successful countries economically in all of history. He took Russia from the very worst of imperialism and brought the people of the worlds largest nation together faster and more successfully than anyone else in history. From the starving and freezing masses of the Revolution to setting the groundwork for a world superpower in 10 years, if only there would have been more time he really could have made the system a success. Mao was also brilliant, yet he became overwhelmed with his power and set China back when he attempted to eradicate culture.
As for the lack of progress that many people mention, it happens, but consider this... The U.S actually feared that the Communist world could destroy them due to their technology for many years. They put a man in space first, and created mass amounts of weapons powerful enough to destroy the Earth. Now I certainly don't think this is a good thing, but they advanced technologically quite well despite a system that is said to not allow that sort of thing. And just think of all the money poured into the space and weapons race, that had it been used on the people would have given the USSR a real chance at great success. There are certainly downsides to Socialism, but it is far and away the best system if worked properly. The only problem is it has gotten such a bad name, who knows if it will ever receive another opportunity with a good leader, but it certainly deserves the chance. If worked properly, all people would receive a standard of life previously unimaginable.