Just got back from a week in Panama (in 2010) with this guide, which was last updated in 2007. Unfortunately, the lack of updated material really showed, as prices were almost always less than they really were, which is important considering how much in Panama relies on bargaining (for a boat to bring you out to islands, or for cabs), so you need to know how much you should pay to avoid being ripped off. Also, every single last one of the restaurants suggested in Casco Viejo no longer existed. There were quite a few things that, while still in existence, the descriptions were definitely "off", making me wonder if they had changed significantly since they were reviewed.
It still had good suggestions for activities, and was a valuable reference for things to do. I think I would advise waiting for a new version of this guide to be published, or at least try to update the guide with information from the web before you go, as it was frustrating at times how ridiculously out-of-date the information was.