This is a similar movie to "The Matrix" and "Dark City". It discusses virtual worlds and artificial intelligence. Important questions in our society. Personally I find these kind of movies to be very interesting and both Matrix and Dark City are among my favorite. The Thirteenth Floor lacks that little extra but is absolutely a well done and solid movie.
The plot is very good. It starts kind of slow but as the movie progresses you will be amazed. I had some ideas to the plot twists yet when everything was revealed I was very content.
What prevents this from being a masterpiece is probably the acting. At times it seems a bit stiff and some of the cast could have been recast with other people. The rest of the film is luckily so good that this is just a minor flaw.
The big variance between the simulated 30's and the real world helps the viewer understand the plot. It also gives the movie a noir feel. The Thirteenth Floor will leave you with some interesting questions and make you think twice about the reality around us. 8/10