You're not the first person This Troper has heard say this. I don't think it's a judgment on the book, just that the movie was that well done.
The second novel, Something Blue, makes Darcy significantly more sympathetic. One assumes the sequel film, if there is one, will as well.
Crowning Moment of Awesome: Rachel is in London and Darcy is on the phone with her, ordering her to get back to New York, but when she says, "Right now!" again, Rachel, having had it up to here with Darcy, says, "No!" and hangs up on her. Doubles as a Crowning Moment of Funny when we get Ethan, Darcy, and Claire's reactions.
Fridge Horror: Darcy is able to get out of her relationship with Dex by getting pregnant by Marcus. Although this allows Dex and Rachel to be together guilt-free, you have to consider that Darcy and Marcus-two despicably obnoxious and selfish people- are going to be PARENTS.
Somewhat alleviated in Something Blue, when Darcy becomes more sympathetic and grows out of her selfish and obnoxious behavior, Marcus however remains such..
Fridge Logic: During the flashback to Rachel and Dex in college together, Dex offered Rachel his only pen when hers fell onto the floor. Why doesn't she duck under the desk to pick them up, or roll them with her foot back towards her and get them from there. From the way it was shot it looked like they were less than a foot away from the edge of her desk.