Vanilla Sky does have a decent payoff at the end that makes it worth watching, but you need to sit through an agonising start for it.
The story is told as a hallucinatory flash back, as though the primary protagonist were dreaming. David Aames (Tom Cruise) has it all. So much so that he plays around with people's feelings, including those of his best friend Brian Shelby (Jason Lee), his fuck buddy Julie Gianni (Cameron Diaz), and his new love interest Sophia Serrano (Penelope Cruz).
David can feel himself falling in love with Sophia even as Julie is stalking him, but his sexual desire wins out in the end and he goes on a fatal journey with Julie. This journey doesn't kill him, but it does deprive him of the one thing he really values. He chooses then an alternate solution to make his true desires come true, but his guilt sticks with him and messes with his dreams (quite literally).
I'm not sure why the beginning was dragged out so much, and why it felt so boring. I gave up watching this film the first time and had to come back to it again when I was in the company of friends and didn't have anything better to do. A lot of good movies (for example, Minority Report) set up the story without that much talk. But once the movie kicks in high gear, which happens after Sophia rejects David, then everything happens almost too fast. The explanations given for David's behaviour are completely rushed, but it results in a giddy "I gotta see it again" experience. The film pretty much makes complete sense the moment David steps into the LD office.
It's quite clever to not make the whole experience that David goes through a dream; instead, almost all of what we see is real (though narrated) except the very end, where his dreams (or nightmares) have really replaced his reality. Since the movie is narrated as a flashback, it's important to keep track of all of David's dreams.
While all the performances were quite good, I thought Cameron Diaz's stood out as the best. There were times I thought she looked positively demonic with a green fire glowing in her eyes. Kurt Russell (as David's psychiatrist) and Penelope Cruz also present decent performances.
Ultimately, Vanilla Sky is more of a morality tale than a science fiction movie (though it can be readily argued that the film simply transcends genres), commenting on subjects like vanity, infidelity, love, sex, desire, choices, and even the medical profession. I liked the non-preachy approach presented here, as well as the suspense created in not knowing exactly what's going on until well into the movie. Definitely worth checking out.