Probably one of my favourite episodes, because it is Star Trek in its purest form. An intelligent idea executed simply but effectively. The special effects budget was hardly touched (the flying exocomps look positively wobbly at times), the actors were given a chance to shine.
At the core of the episode there is the question "When is something a living thing?" For once, Dr. Crusher can do more than just diagnosing a medical problem. When Data has a question about ethics, he goes to see the doctor. In the best tradition of Star Trek, a science fiction story line touches today's ethical problems. (A doctor talking about when something is a living thing comes close to the entire abortion discussion.)
The episode isn't perfect. Especially in the beginning the technical mumbo jumbo can be quite distracting. By putting Picard and LaForge in danger when Data has to make a difficult decision, the writers take the melodramatic route. (Picard is Data's father figure, Geordi his best friend. Would the story be just as poignant if two lesser known characters were threatened by Data's action?) The lack of any court martial after the incident seems unrealistic.
Of course, the star of the episode is Data/Brent Spiner. Remember that Spiner cannot show emotion as Data struggles with his dilemma. As far as "successfully underacting" is concerned, any actor could take a lesson from Spiner.