Frommer's travel guides are usually my go-to resource when planning any trip, so when we decided to take the kids (ages 3 and 5) to Disney for the first time, naturally this was the first book I ordered. Luckily, I also purchased the Unofficial Guide to WDW, and that has by far proven to be the more useful book for planning this trip. The Frommers guide can't even come close to matching the level of detail available in the Unofficial Guide. It also directly contradicts some of the advice given in the Unofficial Guide. For example, on the question of whether to take advantage of Extra Magic Hours (EMH) to avoid crowds: Frommer's says yes, but the Unofficial Guide points out that while crowds may be thinner during the magic hours, they tend to be far greater then normal during the rest of the day, and that this outweighs any advantage from the EMH. They therefore advise readers to avoid any park on its EMH days, unless you're planning to go for the EMH and then take the time and trouble to move to another park for the rest of the day.
In general, the Unofficial Guide takes a more "scientific" approach to its advice - they actually measure crowd levels and attraction wait times on various days, and continuously survey their readers about attractions and dining options, for example. The Frommer's guide seems to be based more on the experience of just the author (though she does seem to be very experienced, having traveled throughout Orlando with her five kids over the years). I tend to value this scientific approach and the experience of many people over that of a single individual but if you are more laid back about your vacation planning then the Unofficial Guide may be overkill. This one is certainly the more approachable of the two.
Also, if you are planning an exclusively Disney vacation, note that this book only devotes a small section to Disney and also covers Universal, Seaworld, and other area attractions. This may be a plus if you plan to venture beyond WDW, but a waste otherwise. In fact, this book takes a somewhat cynical approach to staying at a Disney resort due to the additional cost involved as well as the fact that this makes it harder to leave WDW and explore the rest of Orlando. While this may be true, depending on the length of your vacation and the ages of your kids, this may not be a priority for you.
One thing I did like in this book is the age recommendations for each ride. While the Unofficial Guide provides ratings by age group (preschoolers, grade school, teens, etc), and also provides touring plans for certain age groups, it does not break each ride down to a more specific recommended age. While I plan to use the Unofficial Guide to decide which attractions to visit, I used the Frommers guide to make a note of those that may not be appropriate for my kids due to age. This way I will not try too hard to convince them to try a ride which may end up being too scary, but won't be afraid to push them to try something that should be perfectly appropriate.
All in all, I mostly found this book useful as a quick reference, to look up a phone number for example, which is much easier done here than in the somewhat unwieldly and overwhelming Unofficial Guide. But it is the Unofficial Guide with I studied closely and turn to over and over as the date of our trip approaches.