I have difficulty envisioning exactly how this market is going to develop. None of the products (AppleTV, Roku, Google, Amazon) are differentiated enough to imagine someone who's pretty happy with their box switching, particularly given various ways you'd get locked into particular features or media (e.g., if the average AppleTV user has bought a bunch of movies from iTunes, they're unlikely to switch to Amazon even if they like the games).

At the price point, you can imagine people owning more than one device to get access to different feature silos, but the huge duplication of features (e.g. every device having Netflix) makes this feel wasteful. Furthermore, I think people quickly run into a limitation on how many HDMI ports their TV has. If you're going to have a cable box and DVD/BluRay player connected, you need to have 3 ports to be able to connect one of these devices at all, and you can't plausibly have more than two. (I don't know how many HDMI ports TVs have in the market, but 2, 3, and 4 seems to cover the consumer space in a very small sampling of Amazon availability.)

If I were building one of these devices, I would include an HDMI switch with pass-through so I don't consume an HDMI port. In the absence of that, it's not clear at all to me what the shape of market adoption looks like.