The Marriage Plot shall be a novel of comparisons. The most frequent comparison to be made is with Middlesex, Eugenides' last novel published nearly a decade ago. It's an unfair comparison--Middlesex was epic in many regards, and had Eugenides attempted to recreate his Pulitzer-winning novel he would've failed. The Marriage Plot is meant to stand on its own and it does so admirably.
There shall also be comparisons of fictional characters with those who lived. Does Mitchell share a likeness with Eugenides? Is Leonard based on David Foster Wallace? There certainly are many similarities. Does this mean these two authors chased after the same girl? Possibly, but I don't think it matters. Whether The Marriage Plot is based on factual events or simply borrows a few facts to create this fictional world shouldn't matter. It's a good story. Theorizing can be fun, but it shouldn't hinder the story.
Lastly, I think comparisons can be made between The Marriage Plot and Franzen's Freedom. Despite their many differences (Freedom: decades, Marriage...: one year; Freedom: Suburban America, The Marriage Plot: College with stops in Europe and Asia; etc.), there is a similar plot and atmosphere at work here. It's hard to shake at first. The big difference, however, is that The Marriage Plot is ten times as believable. While Freedom was closer in subject to the average American reader, it was peopled with characters placed in situations that were difficult to identify with. The Marriage Plot was peopled with people, and whether I liked them or not I could still see them as real.
Which brings us around to the divisive issue with The Marriage Plot: Characters. Unlike Middlesex, which was very plot-centered, The Marriage Plot is almost solely character driven. If you like the three main characters—everyone else in the novel is very forgettable—you'll likely enjoy the novel. Without that connection, however, there is little else to hold onto—the plot is simple and the language fairly minimalistic. If you're a hundred pages into this novel and you hate these characters, nothing will change your mind over the next 300 pages. That's not to say that these characters are not hateable—that's part of their allure—but a reader who isn't routing for one character or another is going to find this read incredibly boring. (Note: At times I was on Team Mitchell; other times I was on Team Leonard. Both were horrible choices for Madeleine, but Eugenides pulled me into these characters so effectively that I couldn't help but get sucked into their little drama.)
Add Eugenides' slightly meta fictional twist to the concept of the marriage plot and you've got a winner. No, it's not quite Middlesex, but really, did we want it to be?