Gattaca is a brilliant under-rated piece of cinema that the not-too-distant future will, in retrospect, see it as one of the more outstanding movies of the nineties. It is prolific, stylish, thought-provoking, and one of the few recent science fiction movies that totally foregoes special effects and does it well.
There is nothing about Gattaca that I didn't like. It is a subtle piece of art that reminds of the writing of Ray Bradbury. Technology (the core element of science fiction) is only the backdrop for the story of a man who goes against all odds, including his brother, and overcomes those odds.
Make sure you watch it more than twice. There are many subtle details that you'll miss if you don't (ie, Gattaca's doctor asks, "Have I ever told you about my son?" not even five minutes into the movie, and childhood Vincent falls down holding a toy rocket...) and it's these small details that create a tapestry of cinematic artistry.
The soundtrack is phenomenal. The sets are noir and stylistic, and (thankfully) instead of trying to present a realistic physical future Niccol instead vies for the FEELING of the future: constrained, restricted, and patterned.
Watch it before it's cool to have watched it.