I think that simply because it is Apple it will be successful, Apples brand image is among the strongest in the world and any product they put out is a quality product, for those reasons I think that Apple TV could easily surpass both Fire TV and Roku very quickly.
If there is one single company in the world that can deliver everything they promise, that company is definitely Apple Inc. I think that they will be able to compete with Fire TV and Roku because they know how to develop this kind of projects so i do not doubt it.
Apple TV is a new application that allows access to thousands of clips, shows and videos. It can safely be assumed that apple TV will likely be a huge success. It consolidates material that people want access to. Furthermore, apple is already an established brand so bringing in customers is a non factor.
The problem with Apple TV is the fact that Apple is a very insular company and isn't very compatible with..well, the rest of technology. People are getting tired of apple and their overpriced products, and I think the same trend will follow with apple tv. We will see how it goes.