Maybe series 8 is not quite up to the sublime standard of series 7, but I still think it is one of the best things on TV and well worth the price of the DVD. However, the DVD extras are disappointing, with just 2 audio commentaries (which I found really boring) with the producer and director and two brief (and also in my opinion not very interesting) featurettes of the Colleville explosion and Walker's murder.
The only thing that let the series itself down though in my opinion was the woeful miscasting and dreadful writing of the character of Sarah Caulfield. The woman playing her couldn't do an American accent if her life depended on it (and her acting was not much better in my opinion)! It was almost painful. However, I am not sure if it was entirely her fault or to do with the directions she was given by the writers/director maybe. Either way, her character was very 2 dimensional in my opinion and while I was watching it I kept feeling there should have been some extra (but ultimately, missing) scenes, or maybe some scenes which should have appeared in a different order, which might have made her part - which was fairly important to the central plot - more coherent.
But other than her, the other actors, especially Richard Armitage and Hermione Norris were excellent as always, and the series as a whole - episodes 4 and 7 especially - was a great 8 hours of entertainment.