I've used Quidco since May 2005. So far I've had ~£350 paid to me in cash backs and have another ~£200 in the pipeline.
You can certainly use Quidco with Currys, but your purchase must be made online. Discount codes work as well, but you cannot walk into a store, with vouchers, and expect to use Quidco. How would Quidco know you'd made the transaction? Actually, more than that - why would Currys give any money to Quidco, to give to you, if you walked through the door of your own accord? It's a bit of a joke really, but one I'm happy to take advantage of, but Currys are really just paying Quidco a "commission" for directing you to the Currys website. The fact that you deliberately went to the Quidco site first, simply to get another ~3% off, seems to be ignored in their peculiar business model.
By the way, going marginally off topic, I just renewed my car insurance. Admiral wanted ~£300 - about the same as last year - but I got a quote with Moreth>n for £217. By going through Quidco to get to the Moreth>n website I got a cash back of £75, making a net renewal of ~£142. Not bad for fully comp on a Nissan 200SX with protected no claims
Just a little teaser (You'll see the £75 from February already validated, just awaiting payment to be made. Most of that £40ish in September was for my Canon 40D, from Jessops

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