Whistler's Father is a great Simpsons episode with a well written plot and many hilarious moments. Instead of having a main plot and a sub plot, this episode instead had two plots that were both equally as important as each other, which I quite enjoyed. Neither one felt like filler, instead the writers were putting in the same level of focus in to both plots.
However, I will say that the "Maggie being able to whistle" plotline was very lackluster and we get as much out of it as my description describes. It's a weird concept that the show has the ability to spin on its head and make fun, but unfortunately that does not happen here.
I did, on the other hand, thoroughly enjoy the plot of Marge working for Fat Tony's crew, it lead to several terrific moments, particularly from Fat Tony, Legs and Louie, and was without a doubt the saving grace of the episode.
Great fun. Thoroughly enjoyable, Whistler's Father is a nice Simpsons episode.
Homer discovers Maggie can whistle and launches her baby celebrity career. Meanwhile, Marge starts working for Fat Tony.