Like many things, the more you put into it, the more you're likely to get out of it.
As Medievalist said, if you generate interesting content to drive people to your portal... yeah, you'll get something back.
Me? I mostly joined because I thought book cover pics looked cool with my reviews - and it seemed like a convenience if people could click through, should they be interested in a book. (And, heck, it's free...) I've received all of one payout out of... um... well, it's been a few years. But I've never put that much effort into promotion, so my blog and site traffic is minimal, to say the least. I've read anecdotal evidence of more popular sites making a decent side-income with their links.
So... do people ever click through to buy things? Yes, they do. Should you rely on Amazon Affiliates for income generation? Probably not, unless you're willing to put the time and effort into building (and maintaining) a web following.