Customers should expect their full in-home cable experience, just delivered over Comcast's "locally managed network" as IPTV instead of the traditional QAM broadcasting. That locally managed network bit also means that whatever you're watching on Roku, doesn't count against Comcast's recently-expanded 1TB cap.
It does, however, mean that you'll need a "compatible IP gateway" for access, which Comcast says it will make available soon for cable-only customers who don't already have its modem. You'll also need a cable subscription, of course, so this isn't really a solution for cord-cutters (yet) like the one Time Warner Cable is testing in New York City.
Another interesting wrinkle for this app is the changing world around it. At the time Comcast announced its plans, the FCC was working on a mandate for cable apps that has since been shelved (although not quite killed, yet) by the new commissioner, Ajit Pai. It's unclear how the newly reformed commission will look upon these efforts as compared to other plans, or what rules it might roll out in the future. Still, the new leadership appears to be more inclined to agree with Comcast's view that a "government technical mandate" is unnecessary.
At launch, the beta app will work on recent Roku devices including the Roku Express, Roku Express+, Roku Streaming Stick (model 3600), Roku Premiere, Roku Premiere+, Roku Ultra, Roku 4 (model 4400), Roku 3 (model 4230 and 4200), and Roku 2 (model 4210), while support for older hardware could come before the official launch. Check the Roku store now to install and try it out.