I have taken them. I do think they can help you gauge your skills and they are as valide as you are honest with your self assessment. I would recommend taking one every so often ... they help me think about what i need to do next in my career relative to training or experiences.
John, your comment about being honest during the self assessment is critical. For this following this discussion, the last sentence also raises another question - If you have gone through a professional assessment, how have you addressed your identified weaknesses - any tips, techniques, training, etc?