I have only watched three episodes but I have thoroughly enjoyed them. The first was a slow start and I can see that many will have been bored, put off and switched elsewhere. The shifts in time can be jarring but overall they make dramatic sense: a linear time line would be dreary.
The criticism of the muffled voices is valid, but there is a simple solution which I am surprised no one has mentioned. Watch it on your PC and listen with professional quality stereo headphones. I use Sony MDR-7506. Suddently the voices spring at you clearly and the stereo effect is startlingly realistic. A large decent quality bright PC monitor up close will reveal elements that a distant TV screen would lose.
One initial surprise was that Dorrigo looked to be too young to have achieved the rank of Lieutenant Colonel. Even the intellectually brilliant Enoch Powell only reached that rank at 30 and at 33 was the youngest Brigadier in the entire British Army. So either the actor is too young or he should have the rank of Captain or Major. But that is a minor quibble. This is a fine production.