How valuable is your DR Testing?

Many companies run periodic tests of their IT Disaster Recovery Plans be it annually, every six months, etc.  The question is 'How valuable is this DR testing to the company?'  Does the fact that companies do this mean that their IT Disaster Recovery Plan will work 100% when it is needed?  Does this testing allow IT Management to sleep better at night?  Is the testing actually a valid test? 

Zombie Data provides an independent audit of a company's Disaster Recovery Test Plan.  In advance of the DR Test period, our team will review the DR Plan with the company staff, insuring that we understand the appropriate categories that their systems have been placed in, required recovery times, etc.. We will be onsite with the company IT staff as they perform the outlined steps, collecting system data, etc.  Finally we will then present our analysis to the company management, identifying strengthes and weaknesses, areas of success and areas that need improvement.

Can you identify common areas of weakness in DR plans you have been acquainted with?  Does your firm perform a partial DR test or a complete DR test?  What makes it a 'partial' DR test?

Would this type of independent Audit of your company's Disaster Recovery Test be beneficial to your small firm?