The #1 rule of all user testing is to test with representative customers. Panels rarely meet this requirement; they're composed of people who get paid a pittance to sit like drones and complete online tests. If you're targeting a very low-level audience, then this might be worth a gamble. But not if you 're a B2B site selling to construction engineers or hospital pharmacists. Not even if you're a normal B2C e-commerce site.
For fun, some of my colleagues once signed up with a panel operator. Despite being fully truthful in their responses to the initial questionnaire (which many people aren't when they register for panels), they were assigned to several studies for which they were not even remotely in the target audience. These "studies" are often a form of voodoo usability that generate misleading results.
Even if a panel operator could get you representative customers, automated studies are still a shadow of real usability research because you can't sit next to the user. Direct observation is invaluable, both for seeing the details that would never get reported in a chart, and for gaining the deep understanding of each user's individual behavior.
http://www.useit.com/alertbox/multiple-user-testing.html