Interesting article over at Hitwise - Are online discount vouchers on the decline?
In conclusion it states the online voucher market is still huge and voucher sites are sending a significant amount of traffic to retailers. But the market is maturing fast and is no longer growing at the rate it once was.
You can see how growth has slowed this year following the massive volumes being driven at Christmas time. LOVEFiLM always held back working with voucher sites closely until this year and so far I am impressed by the ability to get a special offer in front of people, with current traffic performing as expected.