Okay, I own a Roku and I'll give you the rundown:
Roku is a box that you use for streaming things like Netflix, Hulu Plus (regular Hulu is not available) Amazon Prime (if you are a member). You can rent shows and movies through Amazon as well. Roku also has other channels, like Crackle, Pub-D-Hub (old movies, like Attack of the Giant Leeches), News (like Russia Today, Al-Jazeera) and other channels like Chow and All Recipes.
Roku does NOT offer anything live, except for news (Russia Today, CNN International and Al-Jazeera). The Disney channel only offers music videos, no shows or anything else. You cannot watch live television on it (except for the news channels I mentioned). You can get free sports scores from the NBA channel but you cannot watch any live sports unless you pay for the subscriptions (and be aware, if you're team is in you're market then you'll be subject to blackouts).
So if you like streaming movies and news, Roku is great. If you need sports or other offerings then you'll need to stick with cable.