John Simm is such a good actor because like all really good actors his acting is invisible. Without him this might not have been so satisfying. (Emilia Fox is easy on the eye but let's face it her acting is barely passable.) Many other reviews complain about the lack of pace but I found it just fine and the plotting and overall writing was good.
The series captured the claustrophobia of Hong Kong, Kowloon and Macau very well which gave the series lots of atmosphere.
Because I'd skimmed a couple of reviews prior to watching the series I expected the last episode to be a disappointment but to the contrary I found the wrap up to be totally satisfying. And it's all worth it for John Simm.