In a recent article in Inc Magazine, a on-line merchant of gift baskets discusses how a link building campaign cost him 4 million dollars. The loss comes both from a direct drop in traffic to his site and indirectly from the money that he had to spend on AdWords to bring traffic while his site was banned.
While link building is an important part of any marketing effort, I would disqualify anyone who is sending bulk emails promoting this as a service. I feel that anyone doing link building in volume is going to raise flags with the search engines and is very likely to get you banned. Sure, automated and/or assembly line link-building may provide dramatic short-term gains, but you have to ask yourself, is it worth the risk.
Google Official Webmasters Blog discusses the legitimate side of link building and provides advice on building links (here).