The strengths of this book lie in the authors' thorough local knowledge of the island. For example, if you're interested in finding hidden beaches, volcanic pools, self-drive advice into the Waipio Valley, and so forth, this is probably the best book for these kinds of tips. They are excellent at detailing the logistics of seeing island sights on your own (exactly when, how, etc.). They also warn about potential difficulties you might encounter, such as tourist scams or signs indicating a private road when the road is in fact public (Waipi'o Valley).
It has some serious weaknesses, however, when it comes to its sometimes heavy-handed opinions on local businesses. One example is the Red Sail Dive operation in Kohala. I signed up with them despite the authors' description: "Groups are held on a tight leash. Get in, get out, let's go, next site. We received no briefing whatsoever." Since they had a desk at the hotel where I was staying, I dove with them, and found this description to be completely inaccurate. I asked one of the instructors about that, and he said that it was like that 10 years ago (certainly not in the five years he worked there). This advice was been repeated in the newest edition, leading me to believe that once the authors form an opinion, they often stick with it rather than re-evaluating it for subsequent editions. It would be more accurate for them to say "When we last dove with them in 1996,..."
Another glaring example for me was their review of the Hapuna Beach Prince Hotel, which they don't recommend. I more often than not stay at 4* properties, and this is one of the nicest hotels at which I've stayed. It's rated #1 out of 70 hotels on the island of Hawaii on tripadvisor.com (based on 229 reviews), so my opinion doesn't seem to be off-base. What's even more striking is that they recommend the sister property, the Mauna Kea Hotel, which is over 30 years older, has smaller rooms, less attractive common areas, and a less attractive beach.
Their restaurant reviews also seemed based on the authors' limited experience. They didn't recommend the Tres Hombres in Kawaihae, yet I found this to be one of the only affordable places for a decent meal in the northern Kohala resort area (where it's otherwise nearly impossible to get a dinner for two without drinks for under $100). They did recommend the Grand Palace Chinese restaurant in southern Kohala which had some of the blandest Chinese food I've had.
As a final example, their helicopter section doesn't even mention Paradise Helicopters, which not only has the lowest-priced volcano/falls tour from Hilo, but also has all-window seat flights (unlike most companies) with 2-way headphones. We found them (and got a discount) through Tom Barefoot.
In summary, if you have a car and time to tour the island, you will probably want to buy this book for its precise and accurate geographical and logistical information. It also has good background information in an easy-to-read writing style. I would be very wary of their opinions on local businesses, however, since these opinions seem to be made on the basis of limited experience. Therefore, this book wouldn't be too useful for someone who plans to just spend a few days in his hotel and not rent a car.