While it requires minimal knowledge of the prior seasons, you do get more out of Miracle Day if you know the story behind the main characters and how they got there. But it does work just as well as an introduction to anyone new to Torchwood.
The 10 episode season is absolutely brilliant – Bill Pullman is one of the most interesting and engrossing bad guys of all time, John Barrowman is as always amazing and all of the acting is top notch.
The premise is very clever and satisfying, dealing with all the ethical complications that arise well and creating an interesting character study for how people would react to there being no death.
All in all a fantastic series and well worth a buy :)