So I did a little research before signing up, and I've seen positive reviews, I've seen it top ranking lists, and that was the deciding factor for me. I tried it, and it worked. It takes a while to get your cashback, but it's excess money for nothing.
Here's an example:
I shop on thinkgeek.com a lot, and they offer 5% cashback in geekpoints, while this website offers and additional 7% in cashback, and my discover offers 1% cashback on every purchase, so with a $400 purchase, I'm getting an extra $52 back in cashback. Now I only buy during sales, so this just makes any and all deals I get better.
It's free to sign up. You just enter your name and email, then click the link to the website you want to shop at, the same way you do with Shop Discover, or any other cashback program.
My recommendation would be to try it for one purchase. If you like it, keep using it, if not, don't bother coming back. There's no loss, but there is potential gain. I don't know if this website works for users outside the U.S. I didn't look into that. You can try it if you want. Again, there's no loss, only a potential gain. I'm sure it says somewhere on the website though about what nations their cashback program is available in.
By the way, check out my feedback and signup date. I'm no newbie. I didn't just sign up to grab some referrals.