I have a Roku 3 and it's pretty much my main television and entertainment device. If you've ever played around with an Apple TV, its pretty much that only cheaper and with more options. You basically download "channels" that are portals into the various services that you use: Netflix, Hulu, Amazon Prime, HBO Go, etc. These channels are all immediately accessible on launch and load very quickly. The Netflix experience on Roku was tremendous.
Honestly its pretty much the poster boy of KISS implementation for streaming and entertainment. The box is tiny as shit, the UI is generic yet easy to use, and the actual OS is speedy. I've gotten great quality streams from it as well. Honestly, outside of a blu-ray player and any game console you have, this is the only other thing you need (or any of the competing boxes like an Apple TV or Amazon Fire TV). And really, my blu-ray player might just get the axe soon.