The first movie had the advantage of freshness, and this one... well, being a sequel can be real tough for movies, since they are often not judged strictly as separate pieces of work, but rather something of a continuation. That is, a movie can be superior technically (acting, directing, writing) but still feel worse, due to lack of innovation/loss of novelty, or too much comparison with the first.
This one sequel feels like a desperate attempt at continuing the intrigue. It does have better acting, but worse dialog (or at least, not as much fun). It feels desperate because now the Cube is like way too advanced, bringing in some rather unfamiliar physics concepts into play, feeling too unrealistic. The ending is weird, leaving me with questions, and me wonders if it was deliberate on the part of the filmmakers, or if I simply am not smart enough to figure WTF's happenin.