I fill those things out almost every day. Rot's right, they generally don't pay much, but, over time, if you stick with it, you can get a decent payout. I'd estimate I got about $300 worth of GameStop gift cards (from two different survey sites) that way last year and around $50 for Amazon from a third one. I've also gotten multiple magazine subscriptions like wyo [they offer far more women's mags than men's though, that said, I currently get Sports Illustrated, TIME, New York, People, Money, Fortune, NewsChina (gotta keep tabs on LoneSage's world you know), Smithsonian, Wired, The Red Bulletin, etc. as a result of survey sites]. Needless to say, you'll make far more money working a normal job than filling out online surveys, but, if you've got nothing better to do with your spare time, it's a way to bring in a little extra cash. Some of the sites are better than others (a few will cut you an actual check if you just want money, while others only offer gift cards or stuff like magazine subscriptions).
P.S. to Rot: There's a woman in the UK whose blog I follow who wrote that she's too lazy to do surveys, but, if you use SwagBucks (one of those survey sites) search engine as your default you can periodically get small five pound Amazon vouchers (I presume that's another name for gift cards in the UK?) for doing nothing but your typical everyday searches.