All three devices assume you have an HDTV with an open HDMI port. Roku offers several models for people who have older, pre-HD TVs, including the $49.99 Roku LT, which works with analog sets.
The major strength of Apple TV is that you can tie it to your iTunes account and make seamless use of your existing library of video and music content. The initial setup via the Home Sharing feature is needlessly obtuse (it fails to mention, for example, that the computer must be authorized before you will even see the option in the iTunes menu), but once you've struggled through the setup, things work smoothly. If there are wireless-network issues, there's an Ethernet port for making a wired connection.
The Roku 3 has similar physical connections — including Ethernet, HDMI and USB ports — plus a microSD slot for adding memory. Setting up the Roku is as straightforward as signing on to your home network and then logging in to the services you use directly on the TV screen.
On the face of it, Google's Chromecast should be the simplest solution: an HDMI stick not much larger than a flash drive, with built-in Wi-Fi. However, it's not quite as simple as it sounds. Chromecast requires a power source, so you have to plug it into an outlet, just like the Roku and Apple boxes. In addition, many TVs can't accommodate the stick easily in an HDMI port (which is often crowded next to other ports). There's a short HDMI cable adapter that Google supplies for such situations.
In our testing, Chromecast also presented some glitches, albeit minor ones. During installation, for example, the app that has to be run on a computer or phone attached to the same network froze several times, refusing to allow us to enter a new password. It was easily restarted, but showed some rough edges, nonetheless.
Winner: Roku. Roku wins by the width of an HDMI cable. If Apple's Home Sharing feature weren't so awkward, it might have taken the prize.