I'm just here to say Cube is in my top 3 movies ever, and I even like Cube Zero, the prequel, as it pulls off the masterstroke of letting you see a couple of the guys who work on keeping the Cube running, yet it doesn't answer all the questions you have.
Vague but I'm trying not to spoiler too much! (Unlike that list of comparisons, come on guys, and if you're thinking of trying the movie, try not to read most of this thread til you have!)
Just because it's worth seeing - Trailer:
The second one is pretty awful tho, and doesn't 'fit' like the prequel.
Prequel Trailer:
Should probably warn people there's a few scenes of gore in there, one particularly unpleasant part in Zero! Yet, it's not about horror, it's about the people placed in a tough situation.