Yup, it is worth, Only one signature alone can get you 500 Real visitor to your site in 1 month. and i think it helps. if your site is web-master related.
I have also read good things about the signature links on DP. Like other people have said, it's all targeted clicks and that's worth it.
Problem I see with signature links (similar with sitewides), is that the search engines expect the incoming link growth on a healthy site to consistantly raise. When links start to drop it's a sign that the site is inactive or no longer relavant. If you deside to stop paying for the signature, you can easily lose several hundred (low quality) links in one shot. That would seem to tell the search engines that several hundred pages no longer believe your site is relavant.
Traffic from sig links can be tracked if you have some stats tracking system like Awstats. Just see the referral number for each month, and you will see the results.
Only reason I would buy sigs would be "getting the message out". Usually forum software generates sigs on the fly so if the person removes the sig from their settings, the link will disappear from the pages which equals no-go for SERP benefits.
Yes, it is good to buy links with related websites who are trusted by the search engines (high PR). However, you have to be careful as Google is starting to crack down on paid links.
Everyone is talking about traffic in here. I don't think its as much about traffic as it is about getting backlinks.