I think Communism in its purest form is one of the most noble sounding but patently unworkable systems ever invented.
The problem with any economic or political system seems to occur when you get people involved in the running of the system.
The only way
TRUE Communism could work is if the entire planning and distribution system was designed, implemented and maintained by a computer system that didn't allow for outside influences (ie:no humans allowed to reprogram or make changes to the system)
Since the people of the world are unlikely to let a computer run every aspect of their daily lives. (ooops excuse me, my PDA just beeped to tell me its time to take my medication and then call my cousin.

)
But seriously, in
ANY system, the main problem is always the 'human factor' and always will be until we evolve to the point where personal ambition and greed are not the overriding factors of many peoples existence.