I also liked how the show continued for a while as nothing was wrong with the crew returning to their duties. Once again Beverly, like in "Remember Me" and "Cause and Effect", is the first to suspect something is amiss.
Indeed. A few months ago I had a TNG mini-marathon with a friend, and, coincidentally, we watched both of those episodes, as well as "Clues". Afterwards, I couldn't help but think, "Wow, that Dr. Crusher is one smart lady! She always knows when something isn't right."
Anyway, I've always liked this episode. It's a neat, well-written little mystery (even if Picard's "Humans find a mystery irresistible" speech was a little ham-handed).
I do agree that it doesn't quite have the same effect after you've already seen it once, but unfortunately that's true of a number of mysteries.