Sky Now is a rebranded (and restricted) Roku 2 LT. It doesn't require a pre-existing Sky subscription and allows you to sign up for various packages (movies, entertainment) on a monthly basis, buy 24hr access to Sky Sports for £10 and also pay for individual movies on demand. You also get iPlayer, 4OD and Demand 5.
If you get the unrestricted Roku, you get the same stuff plus access to many more apps, including netflix which was the clincher for me. I paid £35 from Currys for mine, so still great value.
It is 720 rather than full HD, but picture quality is great if your internet connection is up to the job.
Personally, I can't recommend it highly enough - just be aware that it's not a "play all" media player; it supports a restricted set of codecs and doesn't support local network streaming. If you just want a cheap way to turn your TV into a "smart" set, it's fantastic. And for a fiver a month, Netflix is good value. I guess that Sky movies for £8 is good too (but I decided to choose just the one)