The first time I watched this episode, it had quite an effect on me -- an effect that did not quite persist the second time around. That said, it is an episode well worth watching.
This episode is somewhat Vertigo-esque in that what it appears to be in the beginning -- the Enterprise crew wondering why an old couple would be the only ones to survive a cataclysmic attack on a planet -- is not what it turns out to be. The story proceeds at a measured pace, perfect for this kind of slowly-evolving mystery.
The ending of the episode is quite powerful the first time you watch it, simply because it is so unexpected and yet follows logically from the first 35 minutes. It really resonated with me and apparently one of the other reviewers. The second time - well, maybe I am just too jaded but the intensity just wasn't there.
As usual by this point in the series, Patrick Stewart gives an outstanding performance. But the two guest stars playing the Uxbridges deserve just as much credit. They brought exactly what was needed to their roles.