How bad is it? Meh. It's minor melodrama.
Should you see it? No.
This was based on the 2008 French film of the same name, but lost what minor charms that film had. It's generally trashed because it stars Miley Cyrus, who is not much of an actress. A girl discovers her boyfriend's been cheating, so she dumps him and starts flirting with his best friend, a musician, and she finds herself falling for him. It's kind of fun to see the parade of actresses: Nora Dunn, Ashley Greene, Gina Gershon, Demi Moore and Marlo Thomas - though none of them acquit themselves well. The humor is badly overdone and it looks like the film was written solely with "how can we make the most money?" in mind.