Netflex - Looking for advice

Question for Netflix users: How does it work? I know that I have to buy a box (I'm looking at Roku III), I don't have Wi-Fi but Roku is HDMI compliant. Are movies and/or TV shows charged individually as well or does the $8.00 per month cover the cost of the shows. Any and all advice greatly appreciated. Thanks in advance.

This will not be in replacemen of our cable service but in addition to it as I have a bundled service.