I recently made the leap to Scrivener, and I am in love with it. (And curmudgeon that I am, I don't fall in love all that often, I should note.)
I now use the software for all my writing, with the exception of super-short blog posts--like the one you're now reading.
As a word processor, Scrivener is adequate; but it doesn't really bring anything new to the table that you don't already have available in Word or Apple Pages.
The real power of Scrivener, of course, is it's outlining capabilities. The software's virtual index cards make it so much easier to organize all your ideas in a visual layout.
Yes, you can outline in Word or Pages. I've outlined stories in Microsoft Excel. For that matter, you can outline on a legal pad, or with paper index cards. But trust me, Scrivener enables you to outline far more efficiently than any of these tools.
Should you buy Scrivener? Yes, you should--even if you aren't a professional writer. Even if you don't have any interest in writing fiction. Scrivener's capabilities could be just as easily turned to the writing of essays, business documents, and even personal correspondence. (I'm very surprised that Scrivener hasn't caught on in corporate environments. As a veteran of cubicleland, I can attest to its potential usefulness there.)
Many fancy software packages are expensive. But you can own Scrivener for less that $50 at the time of this writing. So what are you waiting for?