Yes, forum marketing is a big part of my income. Signatures might play a role, but it also depends on your reputation on certain forums. If people know you are a forum member who contributes with valuable input once in a while, they check your products and offers. Pretty common.
I have had success on a few niche forums, but for the most part, your time will be better spent doing other things. When it comes to IM forums and IM related products, don't even waste your time. The majority of users on those forums are CB users also, so even if you're promoting a product they want, they'll just clear their cookies and buy it under their own Clickbank ID. It is wrong and against CB's rules, but most IMers do it anyway unfortunately.
One thing I can tell you and that you might know already, yet I don't know why so many people insist on on this issue. Affiliate links in signatures are useless and believe me I've been there a few years ago (you won't generate sales even if you have thousands of posts simply because it's spam -- the traffic isn't targeted enough if you're simply exposing your link in front of hundreds of members that are basically looking for FREE information) And that applies to most niche forums as well that will ban your account for aff. links in your sig. If someone is still uncertain, simply pick 50 niche forums, create accounts for each one of them (most of them will require 1 week/50 posts etc.. to have your eligible to post your signature link) and start contributing to the forum, you'll surely end of wasting your time. Al.
You need to be giving quality posts to get anywhere with affiliate links in your signature. This way you are building backlinks and traffic and may see a few sales from that traffic or from the serps. But the most niche forums, if not 99% of them will ban users for using affiliate links in their signatures. I think it's better to use your domain name in your signature.
forum sig do work but on a very small scale. it would be best if you outsource that and focus on other more lucrative methods.
I must say i haven't had much success of making sales through my forums signatures, i think i need to do a little playing around with the wording like a lot of people are saying. On another note i do click other peoples signatures on this forum for services like graphics,logos and headers etc....like the ones that say "Great cheap logos only $4 - see portfolio here" now that i will click and look at but maybe that's just me?