Apple TV and its "walled garden" approach makes it a less useful device in my thinking. If you're an "apple-verse" person, a Apple TV may be the way to go. But Roku with over 1000 channels, and the ability to stream from my hard drive, is the way I go for me. I've got three. A Roku 2 XS, a Roku HD, and the current model Roku 2.
Apple TV is more for the look and easy to navigate. I prefer it over Roku and it is just my preference, but with Roku you can do the same things that a Apple TV could, like watching Hulu, Netflix and browsing the web. However, you can now Jailbreaking your Apple TV device for more content and customization like the Roku. It is like comparing the iPhone and the Android, and like dkreichen analogy, it is like a "walled garden" because you are by default locked into tinkering with it.