When we decided where we would go for our first trip, we went out and purchased both the Fodor's and Frommer's guides. We found both to be extremely useful, but in different ways. We also found that both had a couple of shortcomings.
Fodor's Guide seems to present a lot of detailed information about what you are seeing in each location. The localized maps are very good (not Thomas Guide quality, but then what is?). Each section is summarized by a list of 5 reasons to visit a given location, which is very good if you feel overwhelmed by the amount of things to see and do in a given local. Unfortunately, the hotel and dining lists aren't very well organized and also aren't too extensive.
Frommer's Guide doesn't have the level of detail on specific points of interest that Fodor's does, but it makes up for that with a lot of very helpful tips about the area. This is especially true when it comes to precautions and preparations you need to make for an area, such as what you need to do to prevent being a pick-pocket victim, getting reservations for attractions, etc. Also, the dining and hotel sections are very well laid out and provide a lot of options for every area. Unfortunately, the maps they provide for each area aren't detailed enough, and in some cases the street layouts are too difficult to discern or are just not accurate (this was especially true in the Piazza San Marco area of Venice).
We did find some shortcomings that affected both books. The first we ran into was a complete lack of mention about photography. It would have been very helpful if they had listed where photography was not allowed. Also, it would be very useful if they also listed where flash photography is prohibited (and why). We quite often saw camera flashes going off in areas that they were prohibited. I want to believe it is just because people are ignorant and don't realize that a camera flash can actually destroy the paint and degrade the art permanently, but I have a suspicion that a lot of them just didn't care. Most point-and-shoot cameras won't capture a good picture in the low light conditions of an art gallery without flash, and rather than leave the art intact, they would rather destroy it so long as they get their picture.
Another shortcoming was a lack of information about over-the-counter drugs. When you are in a foreign country, trying to explain to a pharmacist what you need (especially if they don't speak too much English and you speak very little of their language) can be difficult, frustrating, and possibly dangerous. You could end up with a medication that you are allergic too, and wouldn't really have a way to find out. Some drug names are very close between one language and another, but others are very different. Also, almost none of the brand names are directly translated. It would be very useful if they would list (at least) some common OTC drug names and their equivalents for that country (such as pseudoephedrine, etc).