The books Ambrose wrote in his last years are kind of problematic. I think I have a higher opinion of The WIld Blue than Don does, but I don't think anybody would suggest that it's the best thing Ambrose wrote.
He had an interesting writing career. He started out as a full-time professional academic (he had a Ph.D. in history), and I happen to like some of his earlier books. (I can't claim to have read all of them.) His biography of Emory Upton is the best thing available on that important military thinker. I like Ambrose's history of West Point, though I wish it had been bigger and more thorough. I've been assigning his Crazy Horse and Custer in my "American military history to 1900" course for almost twenty years now. I think it does a fine, respectful job of describing Native American culture (the author is a big fan of Crazy Horse), without lapsing into sixties-type romanticism (or vilifying Custer, or the U.S. Army). I have the impression that Ambrose's multi-volume biography of Eisenhower is respected as the best on the subject. And I certainly like his book on the Normandy Invasion.
In his later years Ambrose became more of a "popular" writer, with all the good and bad that phrase implies. He started getting big contracts (with hefty advance payments) from big commercial publishers. Those people demand that books be written for big audiences, and to deadlines. (Universities, for better or worse, usually don't. If a professor publishes three or four books in his career, that's considered pretty daggone good. Ambrose published twenty-four.) A lot of academics noted that Ambrose's work was becoming a bit...well, slipshod. I think another problem was that as he got richer, and was constantly confronted by deadlines, he turned over more and more of his work to graduate assistants, and simply quit spending as much time on each of his books than he would have earlier in his career.
Near the end of Ambrose's career some people started accusing him of plagiarism. Many of the examples the critics got had to do with small sentences and word combinations; I suspect the aforementioned sloppiness and over-reliance on grad students were behind most of the alleged instances of plagiarism. I don't think Ambrose was trying deliberately to steal anything.
In cases like this I have to wonder how a particular scholar's reputation would have been affected if he hadn't stayed on the job quite as long as he did. I think Samuel Eliot Morison fell prey to that difficulty, and I think Ambrose did too. Morison's place in the history of the profession would be perfectly solid if he'd quit writing sometime in the early fifties, before writing his biography of John Paul Jones, and Ambrose's reputation would be more secure if he hadn't written The WIld Blue.
But that doesn't mean I think he was a bad historian.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.