I've just finished watching this (will do my damnedest not to spoil for those watching at BBC2 pace) and am really surprised it hasn't had more love. This autumn schedule is crowded with BAFTA-hopeful dramas though, so I guess it's been a bit trampled underfoot.
As somebody too young to remember the kidnapping as a news story, knowing only the 'most memorable/lurid' aspect of it, I've been absolutely gripped by this. I'm not a binger by nature, but I found myself watching one or two episodes a day, needing to know what was going to happen next (and resisting the strong temptation to Google).
It's a fantastic mix of suspense, tragedy, drama and offbeat humour (so a bit like Trainspotting...) Unusually, I found the two different settings - the old man's mansion and Rome - equally fascinating, so no sitting through duller scenes wishing they'd get back to the action elsewhere, as so often happens.
Also - excellent acting. I loved 'Bullimore' the butler. And developed a slightly disturbing crush on the psychotic but very, very attractive Primo 
.