[emile]

Do you find that the students tend to screw up when they try to use idioms?

Either that, or they come up with outdated idioms that they've got from somewhere. One of my North Korean students came up with "sock him in the puss" - oh dear

[susan53]

I think idioms can be very "dangerous" for a language learner. If they're either outdated, as you say, or slightly wrong, they're liable to cause amusement (which may be well-intentioned but can be embarassing) or just meet with blank incomprehension. A more serious problem can be when they're used in a stylistically inappropriate context.
If students are learning English as a Lingua Franca, or so called "international" English, they really don't matter. In fact, they are more likely to impede communication if the other participants in the conversation don't recognise them than to facilitate it. But native speaker English is highly idiomatic (much more so than many other languages) and any learner who expects to have contact with native speakers needs at least a high receptive knowledge of idiomatic expressions.
With my students, who are learning English mainly for business purposes, I teach idioms receptively, but warn them off using them unless they're 100% sure of both their accuracy and appropriacy, or in a situation (eg talking to friends rather than in an important negotiation) where they don't mind taking a risk and it doesn't matter if they say something weird.

[clivehawkins]

I've just bought the Oxford Dictionary of Idioms (ISBN 0194315452)and also Phrasal Verbs (ISBN 0194315436) mainly because I have one group who seem obsessed with learning idioms, even though they'd be better off trying to master the simple stuff first. They're pretty good.

You're quite right about usually getting it wrong and it does raise a smile or two on my part. I try to steer them away from idioms but they seem to have got this strange idea in their heads that this is the key to learning English.

The dictionaries are left in the classroom and they can consult them till the cows decide it's time to go home (or something like that) :-)