For me, it depends on the forum, and how I've written my sig. As a general rule, however, I've found sig links to be a good thing. I don't have them here, yet, as I'm in the process of relaunching my site. One more thing... I've found that links un-related to the topic of the message board tend to generate more click-throughs. Go figure.
Forum signatures are much more important than just backlinks. If you rent a signature spot from a very active member who has interest in your field of interest, than its the traffic that matters. They are huge tragetted traffic generators.
I'm sure siglinks are a LOT more efficient than the AdSense link on top of the forum. You never see this AdSense link as you start scrolling as soon as you access the forum. I'm sure that ad on top of the forum has a ridiculous CTR.