If you're into the genre, at least it's far better than Ozon's "8 femmes", even if it doesn't have the same bragging rights in terms of internationally known stars.
Tough looking Gilles Lellouche is the only one who really sings, and to my surprise, pretty well. Mélanie Doutey does the same role we've seen before, the very feminine, pretty but not very bright female star. Julien Boisselier is pretty good as a Machiavellian character in disguise behind a "sensitive guy" facade. He composed a far more complex character the next year in "J'veux pas que tu t'en ailles" His "Raphaël" was a lonesome playboy, a bit like Laurent here. But this being a romantic comedy, don't expect a lot of character development. Alexandra Lamy is the most beautiful female in the film, but can's sing much. Don't miss her funny "guilty while very sexy" bed scene with Laurent, which by the way shows us glimpes of her great body. Mathieu Rozé is another surprise, specially for those who knew him as Lulu in the popular TV series "Central Nuit". The detective and other supporting roles are also well filled. Don't get me wrong, this film is fun to watch, specially if you're in the mood for light entertainment, but don't expect to be surprised by the plot or to get something believable. This very manipulative film is effective, even if as forgettable as any light Opera/ canzonetta.
I agree with with Michael Hack from Frankfurt on IMDb that: "this kind of music, though sometimes pretty close to kitsch, that is so typical for France. Instead of expressing their feelings, the protagonists sing". Consequently, this film is a good introduction to modern French pop culture, specially for those who can't bear slow French movies. "aurlb from Paris" is right that the dancing while singing is off putting.
Enjoy without expecting much.