Much like Vicenzo Natali's later film, "Nothing," "Cube" uses a highly unusual setup and setting to explore the characters who inhabit it. Seven strangers wake up in the titular cube with no knowledge of how they got there and no indication of what, if anything, they're expected to do. As they attempt to solve the apparent puzzle and escape, we learn what drives them and gradually see how the stress of the situation causes their true characters to emerge. In that respect, as a character study, I found "Cube" immensely interesting and even surprising. The character arcs are believable and well thought-out, and the dialogue supports them. I thought the casting was excellent as well -- clearly, that's a "love it or hate it" aspect of this movie, as the other reviews indicate.
The film's weakness, however, is the setup itself -- it's a great idea, but unlike the characters, it's never developed. (Skip to the next paragraph if you don't want to see potential spoilers.) Who "runs" the cube? Why were these people put in it? Where does escaping take them? These are all questions that the characters themselves ask, but they're never answered. We don't even know the time period (for that matter, we don't know if it's even Earth...). It makes it difficult to know how to feel when the closing credits roll.
It's not uncommon to leave questions unanswered to allow the audience to bring their own interpretation to a story, but it's a fine line between doing that and leaving questions unanswered because the filmmakers don't know the answers either (I get the impression that's the case here). Tough call on the rating...but I'm giving "Cube" 4 stars because by virtue of the characters and what we do know of the setting itself, there are a lot of genuinely suspenseful moments and unexpected twists, and I just plain enjoyed the movie. If you're willing to overlook the undeveloped setup, "Cube" is worth your time.