It is one of the most visually spectacular movies I have seen. The first half is a very good movie in its own right. The second half is mostly narcissistic trash, only periodically compelling, in which Cruise also rewrites the story of his break-up with Nicole Kidman, in what seems to me an unseemly manner.
Most of all, it is a Straussian commentary on Scientology (and Kidman), you can start your research here. I am stunned but not surprised that very few reviews have picked on this angle at all (so far it seems that none have and even Quora fell down on the job). Without such knowledge, the movie makes no sense whatsoever. With such knowledge, the movie is entirely coherent but in some regards more objectionable.
There are also some nice references to other Cruise movies, such as Top Gun and Eyes Wide Shut, not to mention some of the non-Cruise classics of science fiction cinema, including Star Wars and 2001 and Solaris.
I am very glad I saw this movie, but your mileage may vary. The Wikipedia entry is here.