I'm not saying Roku is better than AppleTV, but for a large portion of the population Roku is a better fit. If you aren't tied to the Apple ecosystem, there's little AppleTV advantage and a lot of disadvantages;
- Roku offers most, if not all, of the content the AppleTV offers. (not iTunes, but iTunes content can be found elsewhere)
- Roku offers major content access that AppleTV does not. A lot more free or cheap content. Best example, Amazon Prime.
- Roku offers lower starting points $40 for 720p, $70 for 1080
- Roku offers games
As I have said before, I bought the Roku because I have prime and wanted to watch downton abbey. On iTunes (Downton is actually advertised on the AppleTV home page at $2.99 an episode) it would have cost me cost me $55 to get full seasons 1, 2 & 3. Instead, I put that toward buying my Roku 3 for $98 and got the shows for free with my prime account.
Now if I'm buying a movie or show, it's on itunes and AppleTV all the way, but if I can avoid buying it all together because it's available free on Amazon or another Roku only Channel than I am all for it.