I have not tried the ones you mentioned. I do use fatwallet, mrrebates and BigCrumbs
I have received payments from all of them.
Now, here's how all of these services work. Many companies offer an affiliate program where they will give a company a certain percentage of what you buy of you click on their link. So, let's say HP gives one of these site 5%, what they do is have you go to their site first, click on the link to HP and will give you say 2.5%, keeping the other 2.5% for themselves. In this situation everyone wins, HP wins because you may have searched the site for another related company and found that with the cash back HP is actually cheaper. The site wins because they get money to make a profit and continue operating and you win because you get cash back.
I recommend signing up for as many sites as you can find, but set an order in which to use them based on how they pay out, who has the most sites available, etc. Each of these sites will have different companies available, some will only pay out via PayPal, some will mail checks and some will offer direct deposit or refunds to credit cards.
Some other good sites are upromise, idine and mogl. These three sites offer automatic rebates when you use you credit/debit cards at participating merchants (physical and online), but you do have to associate your credit cards with your account, so they know when you do so. Again, I have received payouts from all three and their sites are safe.