I just returned from 2 months traveling through Costa Rica, Nicaragua, Honduras, Belize, and Guatemala. Because I was covering a lot of ground in a little time, I used the book extensively for the busing information, which I found quite accurate. Bus departure times were much more reliable than I expected, though when possible I always asked around and tried to arrive a little early. Prices overall were also reasonably close, even in Honduras where they include a disclaimer that prices were extremely low during their research due to the drama with President Zelaya.
The biggest inaccuracies seemed to be with restaurants either no longer existing, or not being all they were cracked up to be, so I found it more useful to ask around. With hostels and hotels, I was bummed that lp's "our pick" was often out of typical backpacker price range and many of the lower priced rooms were really talked up. You'll have to learn to read between the lines if you're traveling on a really low budget: "well scrubbed" might actually mean brightly painted and "simple" might mean dirty and the toilet doesn't work.
I loved the idea of the green index, but in practice, didn't use it once. It felt like lp just threw that in there to make it seem like they cared, but didn't really put any effort into it. The 2 page index leaves a lot to be desired. Instead of warning that some hardwoods sold in the markets are endangered, tell me which hardwoods are endangered and which are ok to buy. Same thing with seafood - tell me which are harvested sustainably in which areas. Lp was especially contradictory in this respect, noting that I shouldn't eat conch or lobster outside their seasons in Belize, but constantly recommending lobster dishes in Utila, Honduras (where they have no lobster season and harvesting is never very sustainable).
Another, minor, qualm I had was their budget approximations in the beginning of each country chapter. These are budgets if you are doing little to nothing other than eating and sleeping and maybe one inexpensive activity. I'll admit, this could easily just be my own misunderstanding of how to use the book, maybe that's what lp intended, but I didn't have lots of research time before my trip so I used these budgets as a baseline and it didn't work for me.
Overall, I did find this book extremely useful and the negatives above are meant as constructive criticisms and things I would've liked to know beforehand. Lonely Planet Central America on a Shoestring helped me significantly during my travels, but you'll never fully experience a place relying solely on a book so just make sure you don't expect the world from it and you'll likely be very happy!