The specifics of Dent's thesis are more than covered in other reviews, so I'll focus on considerations that someone might want to know when trying to decide whether to buy the book or follow its advice.
First off, the book is fairly "readable." Although Dent uses charts and graphs frequently (indeed, his methodology is to study past demographic trends to ascertain long term stock market performance), he presents his conclusions in an easy-to-follow format.
Secondly, Dent does an excellent job supporting his central thesis that demographic trends can affect economic cycles. His book provides a well-thought out argument that the US, and indeed, the world economy is going to continue to decline as the result of the deflation of three bubbles, the stock market, real estate, and commodities.
Thirdly, it appears Dent has previously made several significant contrarian predictions that have proven correct, perhaps most notably the collapse of the Japanese stock market in the late 1980's and the tech bubble of the early 2000s. While Dent's predictions aren't always 100% accurate, they do appear to often hit near the mark (with the exception of his prediction that the Dow would hit 40,000, and probably a few others that I am unaware of).
One thing that I find interesting is that, using demographics, Dent not only predicts economic cycles, he explains WHY the economy behaves as it does. In this regard, I find that Dent's use of charts and past cycles is more persuasive that many other authors who simply identify patterns and make predictions based upon them (think of the Superbowl winner predictions, skirt length, or what have you).
The major downside to the book, to me, are the fairly constant adds for "free e-mail updates" - do these e-mails include solicitations to buy Dent's newsletter? I don't know, but I sometimes felt like the book was a teaser to sell more stuff to me. I also felt that Dent's free confession of mis-calling Dow 40,000 cut a couple ways - I appreciated his candor, but his post-facto rationales at points led me to wonder what might be missing from his current book. He's certainly had his share of misses, and it brings to mind the old saying that "even a broken clock is right twice a day."
Anyone considering this book might also want to review behavioral finance - the notion of "availability error" (viewing current events through superficially similar previous events) and "confirmation bias" (we observe events that confirm our hypotheses). Behavioral psychologists note that the use of so-called judgmental heuristics (shortcuts to manage large amount of information) can make assessments of market odds difficult. I can't say whether Dent's predictions suffer from any of these flaws, but readers may wish to consider books like Taleb's "The Black Swan" (one key point being humans see patterns where there are none), Paulos' "A Mathematician Plays the Stock Market" (overview of behavioral psychology) or even Dreman's "Contrarian Investment Strategies: the Next Generation" (extensive discussion of pattern-seeking) before committing their assets as suggested in Dent's book. This is not to criticize the book itself, but simply to suggest that as with anything that purports to predict the future, readers should go in with their eyes open.
I can't say that I'd follow Dent's stategies for investing, but the book at least made me think about how demographics could affect the economy. For that, I'd say its a decent read.