Agreed, if you hire a third party, you can never fully be aware of the who/what you're getting for your dollar. I would point to the most recent mess involving J.C. Penny for an example of what can happen if there is poor oversight. Even if you do pay or hire a "reputable" company, there is still the potential for mistakes if the work is not closely monitored.
Unfortuantely, link-building is a tedious, tiresome, often thankless task!
Before you decide to hire someone, be sure to look at all of your personal resources for linking opportunities: customers, vendors, professional contacts, friends who blog, etc. These might be easier and more reliable sources for potential links than hiring a temp, etc.
