Gates has certainly had a long and varied career in government service. Somewhat of consummate insider, he has had his fingerprints over so many of our countries policies.
Frankly I think he has had a mixed record, when he is wrong, he is very wrong, but when he is right, he can be very right. However to Gate's credit, when he learns about making the wrong decision, he is quick to learn from his own mistake.
The other thing to mention, is that Gates is very effective, especially regarding restructuring the defense department to meet the new realities of a new century.
Nor does Gates crave media attention, preferring instead to get his work done out of the public eye.
My one sentence assessment of Robert Gates would be, as a bureaucrat excellent, as a visionary not so good.
But I find this link a pretty excellent summary of his career.
http://www.tnr.com/article/politics/the-reinvention-robert-gates
And if any find the reinvention phrase harsh, I find it more of a virtue that shows his adaptability.