I own 3 Rokus and like them very much. Great little bit of tech.
If you have HBO through you cable company you can access a bunch of the on demand content through the Roku.
I access all my digital movies through Flixster and the PLEX channel allows you to connect all your downloaded content from your home computer.
You get can access Netflix, Amazon Prime, Hulu, Crackle, YouTube, etc....through it.
Primary weakness of Roku is sports availability. Although if you have ESPN I believe you can access a lot its content through a Roku channel.
FYI ~ I no longer have cable. Rokus for streaming online content, antenna to grab OTA HD signal and a Tivo.