The thing is, though, if the right idiom is used in the right sentence, it sounds and feels good, but as with all good things, too many idioms will make you look like you're trying to hard to impress your audience. And French people don't like this -- it comes off as arrogant to them.
The same caution applies to slang in general. Yes, French people do use a lot of slang: "merde" (shit) is common, so is "con" (asshole, stupid...), "putain" (bitch, whore), and other less-than-charming words. But what sounds perfectly normal from the lips of a French person, sounds contrived coming from a foreigner.
Furthermore, slang is used in a certain context; if you use it with the wrong people, or in the wrong situation, it will be a big faux pas.
If you like slang however, check out Olivier's series of post about "Le Parler D'jeunes": the slang of the young French generation.
