Frommer's is packed with information, and it's reasonably well organized. It covers a huge and diverse country with a reasonable amount of detail. It's written to appeal to a wide range of travelers, but it's probaly most useful for people who are (1) traveling on their own rather than on a group tour and (2) planning to go mid- to high budget, not low-budget. (Mid-budget in India will get a you a long way -- food, services and rail are amazingly inexepensive as of March '07.)
There's a bit of hysteria in the section on getting ready -- you don't need all those immunizations unless you're going to places where the specific disease is endemic. Check on-line with the US Centers for Disease Control for more realistic advice.
In contrast, the authors are right on target when they warn repeatedly about minor scams. From the moment you go through customs you will be approached by people who have some offer that's a scam or semi-scam. Even when you hire a good guide from a good company, there's a high probability that you'll be taken to a shop whose owner gives the guide a commission. It's endless and ultimately an amusing game, and Frommer's does a great job of hoisting the warning flag. Pay attention - from the second you arrive.
The book also warns against trying to drive yourself in India. I've driven in places like Sao Paulo, Bogota, Rome and Mexico City (and Boston!), and I agree with that warning. Driving in India was scary 25 years ago and it's even more so now due to the great increase in traffic. There are clearly rules of the road, but you'll be dead before you figure them out, so hire a driver locally, fly long distances, and take the train inter-city when flying doesn't make sense.
The discussions in the front of the book about what's good and what's great are excellent and valuable in helping you decide how to spend limited time in India. The suggestions are divided into rankings of sites and rankings of "moments" or experiences. In the back, theres a short but useful section on Indian history and on its multiple cultures, knowledge that you can use to put what you see into context.
The book's one major fault is the lack of good city maps, as mentioned by another reviewer. Even the state maps are poor in terms of identifying good roads and main rail lines. There's a reasonably good explanation of how the rail system works and it tells you what class of travel to book, but that section would be improved with a clear description of what you get at each level of service. (The top level of overnight train is 1 AC which is a 2- or 4-berth air conditioned compartment, and the bottom level is Sleeper, which is a bare bench in a wretchedly crowded coach.)
The book is organized by state and region, and two places that could be twelve hours apart may follow one another in the text, with no indication of their separation. It can take an hour or more to go 30 miles on a main highway; a table of time between major points of interest would help readers plan a realistic itinerary.
Despite these shortcomings, Frommer's India is a solid guide packed with useful information. Not perfect, but very good.