I'm planning a trip to Scotland (leaving next week), and have never used Frommer's guides before. It has good suggestions for interesting touristy places to visit in Scotland, and gives a well written, concise historical description of the sites, which I like.
However, for booking hotels, I haven't selected a single suggestion from Frommers. This is because their suggestions are generally way too expensive. For example, looking for accomodations in the Orkney Islands, you'd think it was impossible to spend less than $150 a night if you relied exclusively on this guide. On the web, I found that there are at least a handful two and three star hotels in Kirkwall that were less than $100 a night, and look very nice.
Also, it says in Frommer's that it's ~$100 to travel by train to Edinburgh from London, but it doesn't mention that there are actually a huge range of fares, from ~$20 to more than $200 at www.virgin.com/trains, that depends on how early you purchase the tickets, and how flexible you want the ticket to be.
From these and other examples, I've decided that Frommer's is a fine place to start for an overview and figuring out where I want to go, but I'd end up unnecessarily spending a fortune if I relied on it for the practical stuff.