There's a bit of a fan view with Sapphire and Steel that the odd numbered stories are a bit pony and it's the evens that are the ones to treasure. By and large I don't disagree although I do think the 2nd one is overlong. I do have a soft spot for 5 though. It departs from the formula a touch in that it has a larger cast and does feel less claustrophobic as a result, and is also an obvious pastiche of the Agatha Christie murder mystery type story but I think this works in its favour. Indeed, in the way that peak Doctor Who could take established gothic horror tropes and stories and put a distinctly 'Who' spin on them, so this shows that S&S could quite easily have done the same on a more regular basis.
The supporting characters are largely well drawn and acted and there is some wonderfully waspish badinage between them. It does really feel like a hellish party populated by a group of people who have to tolerate each other regularly at social occasions but can't stand the sight of each other.
The denouement is suitably oblique enough to confuse and put off the casual viewer while pleasing the die hards.
And to top it all it was largely written by Don Hougton who was responsible for one of the all time top 5 Doctor Who stories, Inferno.
Really, for enthusiasts of this type of thing, what's not to love?