The primary plus for this movie is the combination of Chris Farley and David Spade. This was the first film in which this comedy duo displayed their Laurel-and-Hardy-esque brand of humor. Farley's obvious physical comedy skill is perfectly augmented by Spade's sarcastic take on every situation they find themselves in.
This movie stands apart from other comedy movies. Tommy Boy ranks in with Blues Brothers as a comedic work whose individual scenes and bits stem from the plot, rather than serve to break up the storyline and give the film a disjointed feel as so many other comedic films do. Thanks to excellent direction by Sagal, every scene is tight and immediately foreshadows or acts out the story being told. He is as much to credit for the film's hilariousness as Farley and Spade are. The acting of Dennehy, Lowe, Ackroyd, and many others (see the gas station attendant 22 miles from Davenport) strengthen the film.
The film warrants multiple viewings because there are many fine nuaces to the film that may be overshadowed by Farley/Spade.
I believe this film is very well-made and is THE funniest movie I have ever seen.