Really, being able to stream Netflix is the biggest reason to have a Roku. I've used a few different devices that did NF, and researched alot more. Roku probably has the easiest interface, along with its ease of operation. My Wii has NF, and even though I really like the interface and browsing options, using the wii mote is a pain. Others like the Sony only give you access to your IQ.. My WD Live Plus has an interesting Netflix app but doesn't work as well as the Roku (I dont think).
In addition to that, other channels like Hulu + and AVOD are the only ones worth it to *me* to pay anything for. Free stuff like the youtube private channel, PODTV, CDNtwo, Itunes Podcast, and even Pandora are that tasty cream cheese icing on the cake
Streaming is in its infancy, and of what's out there right now Netflix is the biggest reason to buy any unit. Down the line those choices are bound to improve though.