Leonardo DiCaprio and Kate Winslet play Frank and April Wheeler. The Wheelers are having major problems in their marriage, and an aspect of the movie that I thought was very well done was the ambiguity regarding whether Frank or April was the "bad guy" who was bringing the other one down. At certain times I sympathized with Frank, and then I'd turn around in the next scene and think he was a complete jerk.
There's also some ambiguity as to whether April has a mental illness or just a negative attitude. Kathy Bates plays the Wheelers' realtor/friend, and there is a very powerful scene where Bates's son (who is institutionalized but out for a day visit) seems to have more insight than the Wheelers themselves about the problems in their marriage.
It's interesting that I seem to like Leonardo DiCaprio in most of his roles (The Aviator being his best so far). Kate Winslet bordered on over-acting in this movie, and based on the interviews with her I saw during last spring's awards season, she may be a bit arrogant. Between this movie and American Beauty, Sam Mendes is definitely tops of the current filmmakers when it comes to examining suburban angst.
The milieu of the movie -- particularly the office scenes but also the way the characters spoke and the male/female dynamic -- was quite reminiscent of Mad Men.
It turns out that the movie is based on the novel, of the same name, by Richard Yates. The novel was a finalist for the National Book Award in 1962 and Kurt Vonnegut called it "the Great Gatsby of my time - one of the best books of my generation."