Today, Apple launched their new Mac desktop App Store. Think of it as a way to buy apps for your home computer. One interesting development is that Aperture is only $79 when purchased through this store. This is a big savings over the $199 in-store/physical box purchase price. I can't see any particular downside to purchasing it through the App Store, except that purchases are tied to your iTunes account. So, if you have multiple machines, you might have to login with your iTunes account on them to download the app.
I'm more of a Lightroom person myself, although at $79 it does make it worth playing with the trial. To get started, just update your Mac to 10.6.6 via Software Update, and you'll see the new App Store icon in your dock.