Robert, I've signed up for Mr. Rebate and so far the only "spam" I get from them is a periodic email telling me to cash out my rebates! I haven't actually cashed anything out yet since I don't like paypal and am having issues with the Post Office denying me service (mainly my fault, i NEVER get mail sent to me beyond pounds upon pounds of junk mail...).
So I can't speak for if they actually pay you, but given they keep reminding me to get paid I take that as a positive sign.
I think they basically get paid on commission from purchases linked from their site and the pay scheme works out such that they pass that commission onto the members. So very not scammy aside from the entire point of marking up products specifically to offer kickbacks and such...