Because Data was the most rigidly ethical character on TNG, the idea of him turning to evil appealed to both audiences and the show's creators. And perhaps the most memorable such turn came in the form of Lore, Data's "evil twin".
This episode adds an intermediary to that yin and yang – the androids' creator, Dr. Soong. Soong isn't a demented evildoer like Lore, but neither is he an upstanding goody too-shoes like Data. Instead, he is a completely amoral scientist, pursuing knowledge without thinking (or really, caring about) the consequences. The interplay between the three – and in particular, the twist near the end – makes the episode worthwhile.
Of course, the most noteworthy thing about the episode is Brent Spiner's tour-de-force portrayal of three different (very different!) characters. The B-plot about a brotherly prank gone wrong was intentionally included as a foil for the main brotherly story, but it comes off as filler.