When I first started shopping I joined shadow shoppers, and it was good for me. It gave me the start that I needed and still sends me new leads and company names to sign up with. Some of the companies are on Sassie and volition, but there are more that are not. If you have been shopping for more than a couple months, I don't think it is worth the money, but if you are new to shopping I think it is money well spent.
Besides Volition and Sassie, there is walletbosters.com and mom-mom.com/mystery two really good sites that have plenty of free to join companies.
Mike