I think it is a great way to earn extra cash, from buying things you would normally buy.
I have earned nearly £100 cashback this year. Most of which comes from Boden 
There are especially good cashback rates for insurance policies on Topcashback - £60 for Sheila's Wheels car insurance. Plus right now Tesco are offering £10 bonus cashback for new customers.
I prefer Topcashback because unlike many cashback sites, they don't charge any admin fees.
If you haven't already signed-up, I really recommend it:
With this link I can earn £1
With this link I earn nothing