If you like B list or older 2nd run movies or sitcoms you'll prolly like what Netflix/Hulu has to offer. The Roku box is just the interface that will allow you to stream those services. Other options are Blu-ray DVD players and TV's that have the same capabilities built right in.
I looked into it and tried Netflix's free trial. We streamed through a wireless BR DVD player. Wasn't for us as we aren't big movie or citcom watchers. Newer releases aren't available to stream and the movie selection is OK at best(Netflix, don't know about Hulu). Netflix has a plan which you can stream and have actual DVD's of new releases, but have come under fire for increasing prices recently.
While very appealing from a price standpoint, is wasn't for us. No live sports was a big problem. I considered adding a digital antenna to get the over-the-air local channels that would help fill in some of the gaps, but still decided it would require too much of a shake-up of our viewing habits.
Still, very appealing from a price perspective(didn't I just say that?). Research the different services and what they offer to see if it fits you, or if you'd be willing to make changes in what you watch.