One thing to consider about "custom" interfaces is whether you really need a custom interface versus a standard Roku UI. For frequent users of smart phones (I'm only guessing here because I don't have one), I would think the users would expect a similar interface across all apps that run on that platform; it makes jumping from app to app a lot easier and provides a much smoother, consistent user experience. If you're trying to mimic your smartphone interface on the Roku, then you need to realize that the same philosophy applies to the Roku: users expect a consistent look-and-feel across all their Roku channels for the same reasons that smartphone users do (and by 'users' I'm referring to myself).
Imagine having a dozen or so Roku channels (or smartphone apps) and having to get used to a dozen different custom interfaces because each on the interfaces' designers wanted to make their app look like an interface they supported on another platform. I suppose it would be okay if your users only used your app on their Roku, and they're buying a new Roku specifically to use your app via their TV, but if they're existing Roku users then they're most likely already used to the standard Roku interface.
I also believe that on the Roku, probably more so than on a web site or smartphone, what's really important is the content delivered rather than how slick the UI looks (except maybe for games). Users generally find something they want to watch or listen to, then sit back in front of the TV to enjoy it. The key here is to make it easy for them to find what they're looking for.
Just my two cents' worth.