i've been a part of the market research they do in malls a couple times when i had some time to kill. Generally paid me enough for a good cappuccino
i've also known people who did the 'mystery shopper' thing and they enjoyed it a bit.
My main concern was the paying to work thing. most or the pay sites say that you're just paying for the lists they've accumulated, which makes me question it even more, but if $20 saves you the time of signing up with hundreds of companies, i'd call it a software expense and call it a day. i'll check with the BBB on a few and see how it goes.
it isn't a serious concern, honestly, it's just one of those things i could do while online not making money