Hi access, understand your concerns, does look iffy but have to say been using Quidco for about a year now and had £360 back for various deals. Best has been car insurance, got 2 cars insured fully comp for under £100 net. Latest has been £60 back for a mobile phone contract and £70 pending for signing up with E-On.
Only 2 failed transactions so far out of 50 transactions, one from Carphone warehouse (supposed to do the deal totally online but had to phone up due to payment glitch) and another for a clothing company for £1.60, haven't bothered to look into it.
Lots of transactions for DVDs etc for pennies but all mounts up.
Trick is to find cheapest deal online then see if a cashbackdeal is better, sometimes the cashback deal isn't as good as just buying in a sale etc..
£10 membership fee comes out of your earnings so you don't notice it.
Haven't used other cashback sites so can only speak for Quidco experience.
By coincidence car insurance renewal forms arrived today. Via Quidco will get 3rd party, fire and theft for £91 less £70 cashback from RAC so £21 net. Result!