I'd say that a heck of a lot of them are not legitimate and are outright scams. But the genuine ones are lucrative. You just won't earn a passive income. For your case, it might be legitimate, but some of these survey works will only pay you once you've completed a HUGE number of surveys (say 500 or so). And sometimes that's only mentioned in the fine prints. Or at least that's my friend's wife's experience.
WRT Helium: Go for it. If you're concentrating on studies or other work, go slow. But if you're not doing anything much, then definitely go for it. Even if you don't get well-rated, the passive income does help if you choose your topics wisely. Some writers there earn over a hundred dollars in passive income a month with just several articles/works.
Before you join Helium though, be aware that there's an invite bonus. This means that you can get a total 5% increase in earnings if you join in pairs. As in the person inviting will get a 5% bonus. Hmmm... I'm not explaining this well... Let me look around for a bit.
*** From the site itself ***
"Invite-a-writer ™: Let your smart friends do the work for you. “Invite-a-writer” is a way for you to spread the word about the great communal writing spirit at Helium, to help the site grow and to earn more money. For every person you invite who registers and writes, you earn a 5% bonus based on his or her article earnings. The more successful invites you have, the greater your Invite-a-Writer earnings. "
*** End Quote ***
It's not stealing the earnings from your friend or person you invited. You just get a bonus, that's all. I'm just saying this because my friend who showed me the website didn't expect me to join so fast. And now that I'm earning, he's slapping his forehead

...
I have to do a disclaimer again though. It's not easy money. Just another way to make fairly good money for those with some talent in writing. And a way to make slightly more than pocket change for almost anyone who can string a decent paragraph together.
Another thing to note is that Helium doesn't require exclusive rights as of the time of this post. Some websites do require exclusive rights though, so if you're a newcomer to online writing, this is something to be aware of.