What are your thoughts on free reward sites?

Lately I’ve noticed there are a lot of websites like this: you sign up on the site; you complete offers(both free and purchase needed), surveys, and possibly do other things to rack up “points” that essentially have no value by themselves; and you redeem these points for items of actual cash value, or even straight up cash!
Many of them reward you for referring people to the website by giving you a share of their points, making this sort of a pyramid scheme. People with enough popularity to get a ton of referrals make loads of points this way without doing anything.

Sites like this can be themed (personally I see a lot of sites for gaming): Prizerebel, gaminglagoon, even Nexon’s official site has a section where you can “Earn free NX”
Or they can just be general stuff: Swagbucks, Rewards1, Points2shop

This raises a few questions. There’s no doubt that (at least some of them) “work” and people have gotten prizes. Do you think it’s worth the effort? Are sites like these are legit, or fraud(and how)?

Do you use a site like this? Which one(s) do you think rewards you the most for your effort?

Do you have any success stories? Or fail stories?

Anything else you wanna say? Tips? ;b