I've traded links with a few sites, but I'm with iaviv; I'll only do this with comics I like. If I'm going to give someone a personal recommendation, I want it to be honest.
I have sites on my Links page
that I link to just because I like, but I haven't made the creator aware that I'm doing this. Anyone listed as an Affiliate links back to me. So my idea is that there's someone I link to, and they start linking back to me on their own, I'll bump them up to affiliate and give them a little more space on my page. If they stop, I'll move them back to the text list, or maybe remove them from the page completely. But if they never add me back, they can be lost in the text list.
It's more about supporting people/comics I like than traffic for myself. But since I appreciate it when people link back to me, I want to highlight them.
If someone wants to email me and ask about being affiliates, I'll consider it, but I need to read their comic first. I typically don't approach others about this, though.
Has it benefit me? Sure, I guess. I don't get a lot of traffic from those links, but I don't have very many readers, so I'm probably not giving them much, either. I don't want to discount that it has helped, but at the same time, it's not like I'm swarmed with new readers, either. One new reader every now and then is plenty.
I also have a link to my favorite stock photo artist's dA account on my page. It's very unlikely that she'd ever find out about it and I get absolutely nothing back from it, but I like her collection a lot and she's very pleasant to talk to, so why not give her some more attention.