IMO Roku by itself is not a cable killer since Roku lacks some key features like flash-enabled web browser to access content not yet available (Roku channels) created for the platform. Nonetheless, Roku has the most content available of any streaming OTT brand out there.
BUT...When you combine Roku with:
1. Google TV device
2. OTA antenna OR QAM/Unencrypted basic cable (I prefer QAM since my ISP is my local cable co.)
3. PLEX and PLAYO** (when using supported and unsupported scripts/plug-ins)
4. DVR Solutions (EYE TV, Simple.TV, HD HR with Media Center, TiVo)
5. "AirPlay"/Casting Solutions for Roku and Google TV
6. Subscription services (Netflix, Hulu+, Amazon Prime, Aereo) $30 or less per month depending on which you decide on. These services are to get access to content simply and legally if you don't want to deal with the hassles of complicated hardware setups and using questionable file-sharing alternatives.
1. Google TV provides a flash-enabled browser and the best YouTube experience that the Roku lacks along with other "Made for Google TV apps" available in the Google Play Store. GTV also has an HDMI pass-thru connection that will allow you to connect your Roku to it and allow for PIP and you can also control your Roku with the Google TV remote or keyboard. I have a Logitech Revue with Google TV (Android 3.5) and I control my Roku with the wireless KB of the Revue. Some sites and broadcasters block Google TV but the content blocked can be accessed via other means like Roku, OTA or PLEX and PO.
****For a limited time PLAYO* is FREE for Google TV devices so you can use all PO features free on GTV. PLEX is also available but the app is $5, but well worth it or use PLEX free through your attached Roku*****
2. An OTA antenna is great if you get clear reception and you live within a 30 mile radius of TV towers. But not everyone gets good OTA signals or can install an antenna (Like in my case) so I use QAM instead. Because my ISP is my Cable company, I can connect my line directly to my TV's that have built-in ATSC/QAM tuners to get all my locals free and with perfect signal quality and no need for an external antenna. To avoid having so many coax cables running through my home I only have 2 cables running. One goes to my main LR TV the other goes to my Mac Mini (Media Server for DVR/PLEX/PO) to its EYE TV tuner. The other TV's in the house are connected to Roku's. I watch all my DVRed recordings on all my Roku's through PLEX. If you have DSL or Satellite you can look into subscriptions to Aereo and USTVNow or if you have a computer or GoogleTV box you can watch local programming free online through FilmOn
3. PLEX and PO offer both supported and unsupported channels/plug-ins that give you access to pretty much any show or movie out there. Some of the providers are questionable file-sharing sites but....the content is available to access free if you know where to look and what plug-ins to use.
****It is up to you to look into this, please do not ask me how and what sources****
4. Some people use Simple.TV, HD HomeRun or Tivo. I prefer EYETV with Nowhere DVR channel and PLEX for my DVR solution. Main reason because there is no premium EPG service to pay and I can access my recordings on all my Rokus. HD Homerun with Media Center also offers free EPG but unless you have a cable-card Prime HD HomeRun you cannot use it with Roku on a dedicated channel. The new Roku HDHR channel does not work with their OTA/QAM tuners. It may still work via PLEX but I don't have that setup (yet) to confirm if it works or not.
I record to an external USB HD connected to my Mac Mini and pointed PLEX to use a shared folder to it that allows me to access my recordings from all my PLEX supported devices. You can also unplug the external hard drive and connect it to Roku 3 and Roku 2XS boxes and play the recordings using the Roku USB Media channel since EyeTV can convert the files into iPhone/iPad video format which is supported with Roku's built-in codecs.
5. "AirPlay/Casting Solutions": Roku has Bleess, PlayTo, Twonky and TrimePlay and PLAYO* has also a new casting feature. More channels/options are being created and supported for Roku and Google TV (MiraCast, DIAL, etc.). More recently Sling Player released an "AirPlay" channel for Roku as well to play on your TV via Roku box. So...no need to look into AppleTV or ChromeCast players if you already own a Roku or GTV box for this feature.
6. Although you can access all your content free from other means. It is much simpler to just subscribe to Netflix, Hulu+, Amazon Prime and Aereo (If you cannot access OTA/QAM). If you subscribe to all 4 services you would be paying $30 per month instead of $50 or higher with a TV provider. Because of my setup I only pay $8 for Netflix but I can cancel that too if I wanted but for simplicity and legal reasons I only use questionable means at a minimum.
7. Many public libraries have free movie DVD's and TV series. Watch free content at your leisure. No need for DVD kiosks unless you want the latest and you don't want to rent from Vudu, Amazon, RedBox, BB on Demand. You can also rip your already owned music and movies onto your media server and view via PLEX.
Bottom line...there is a Cable Killer solution for cord-cutters and cord-nevers but Roku is just one important piece but by itself it is not a "Cable Killer". When used to its max potential in combination with the right equipment (Google TV and PC as media server for local media DVR, PLEX and PO, OTA/QAM and DVR) and subscription services then and only then it can achieve this title.
Hope my tips and tricks help you guys achieve "cutting the cord". Have fun experimenting, doing research and saving $$$$.
(2) Roku2 XS,Roku 1(New),Logitech Revue Google TV,Raspberry Pi+XBMC,Mac Mini HTPC-EyeTV DVR+Rabbit TV+PLEX+XBMC,TiVo Premier,Wii,PS3,Sony Blu Ray w/ streaming apps,Equiso Android TV
Local TV: QAM from ISP coax line. PLEX+unsupported plug-ins.