You'll need to subscribe to Hulu Plus to view on a Roku. We love ours.
However, as mentioned, there are some titles on Hulu via your browser (computer) that are not available via Roku. In as much as they both bring the Hulu subscription into your home through the internet the Hulu system can detect the difference between a computer's browser and a set-top box (like Apple TV, Roku, etc.). Those boxes have some things blocked.
We cut the cable a couple years ago and between Netflix, Hulu+ and Amazon there's pretty much plenty to watch. Just keep an eye open for if your internet provider likes to either throttle your connection (slow it down) or cap your data usage. You could find yourself with crappy "reception" or just a big overcharge on your bill.