I've been mulling around this review for a while (re: a week) and finally decided that Joe Neumaier of the New York Daily News summed it up exactly as I would have:
"...[the film] comes close but falls short of capturing Richard Yates' terrific novel... the movie — two-thirds Mad Men, one-third American Beauty, with a John Cheever chaser — works best when focusing on the personal. Thankfully, it's there that Mendes and screenwriter Justin Haythe catch some of Yates' weighty ideas, and where Leonardo DiCaprio and Kate Winslet succeed in doing the heavy lifting... DiCaprio, round-shouldered and sleepy-eyed, and Winslet, watchful and alert, raise up each other and everything around them. Never once shadowed by Titanic, they suggest, often wordlessly, the box the Wheelers have found themselves in. Whereas the novel is told mostly from Frank's viewpoint, the movie is just as much April's, and Winslet, whether fighting back or fighting back tears, is sensational."
It's like American Beauty on downers, basically, with lots of 50's flair. I enjoyed it, but it is hardly revolutionary (pun intended) for Mendes.