Renowned as much for its profuse and prolific profanity as the brilliant performances of its ensemble cast, Glengarry Glen Ross is one of those movies you see referenced and parodied everywhere to the point that watching the original is almost irrelevant. But I say 'almost' because the writing and acting is so consistently good that, like the similarly over referenced/parodied Pulp Fiction, if you can get past the fact you already know a lot of the things that happen and are said, you'll be delighted by one of the best movies ever made.
I think another good point of comparison for Glengarry Glen Ross is Full Metal Jacket, insofar as both films peak with their opening scenes. The boot camp sequence that opens Full Metal Jacket is definitely the best known and beloved part of that movie, and the drill sergeant played by R. Lee Ermey steals the movie even though he isn't around for the last 2/3 of it. Similarly, Alec Baldwin's legendary tirade against the loser real estate agents played by Ed Harris, Jack Lemmon, and Alan Arkin all but steals the entire film and makes the remaining hour and twenty minutes seem irrelevant because you feel like there's no way any of them can impress the sort of guy who swings literal brass balls at one point and liberally calls the men "fucking faggots."
It's always great to see a film where an older actor is peaking and a younger actor is just starting out, and Glengarry Glen Ross gives us this in the form of the above pictured characters played by Kevin Spacey and Jack Lemmon. You can see the seeds of outstanding future Spacey performances in Se7en and American Beauty in the uptight, deadpan office manager John Williamson.
And of course, Jack Lemmon's desperate older salesman character inspired recurring character Gil Guderson on The Simpsons. It's worth noting that Lemmon himself voiced a similar character in the episode where Marge starts the Pretzel Wagon business. You know, the one with the Asian mafia fighting Fat Tony's gang at the end--"But Marge, that little guy hasn't done anything yet. Look at him! He's gonna do something and you know it's gonna be good!" Anyway, if you've ever known someone who is pushy in an upbeat way and just won't take "no" for an answer, you'll delight in Shelley Levene's descent into madness and utter doom.