Right, I guess it pretty much depends on the product. BUT, the Avangate guys which recommended me several times to start using their affiliates network for my product, they also said that there are quite a few things that you can do to make sure that your product gets to the right market.
For instance, when a website, blog or whatever, requests you the right to become an affiliate, you get to choose if they are right or not for you.
I have also heard about good sales made through an affiliate but that was, indeed, for a large consumer market product. My product being a little bit of a niche one, I haven't been that compelled to jump into such a deal until now. But I will probably give a try as soon as I'm done with other marketing techniques. :-)
Looking at Joel's answer, it seems that he was bit against affiliate programs mainly because the hassle that comes with. A payment processor like Avangate takes this off your head so you don't have to worry about anything regarding affiliate payments and all. Of course, they take their share, too.