Sphere is a movie with an interesting concept. The movie is structured in such a manner that the unfolding of the plot (which isn't completely clear until the end) is crucial to audience interest, and therefore, it's hard to talk about the movie without giving away some of the story. (This means there may be spoilers ahead.)
Essentially, a old space craft has been discovered 1000 feet under the Pacific Ocean. Given the growth of the corals around the craft, it's believed to have been buried for over 300 years. A team of investigators, psychologist Norman Goodman (Dustin Hoffman), biochemist Beth Halperin (Sharon Stone), mathematician Harry Adams (Samuel L. Jackson), and astrophysicist Ted Fielding (Leiv Schreiber), go underwater to investigate.
During the course of their investigation, they discover a sphere in the spacecraft (which originates from the future) that has the ability to make all their dreams manifest themselves into reality. As they struggle to survive and make their escape to the surface (instead of just dreaming about it), their own imaginations begin hindering their efforts.
The idea of what happens if humans suddenly came upon the ability to make every wish come true has been handled more elegantly; in this movie, this issue is just touched upon at the end. I personally think the movie could've been better off if it wasn't the sphere (and the characters' imaginations) that were responsible for the disasters. The initial hypothesis offered by the investigators, the the sphere was a living sentient alien which had gone mad after 300 years of isolation, was far more plausible.
The acting is decent, but the special effects aren't anything you'd write home about. I'd say it's worth the matinee fare if the concept of a wish-granting sphere appeals to you; otherwise, I recommend some time reading Stanislaw Lem.