Compared to all the other Lonely Planet guides I have used in the past, the Portugal edition is basically useless! I regularly use LP guides as I find that their recommendations are usually fantastic for things like hotels and restaurants (as a foody this is particularly important), as well as getting inside tips on unseen sites off the tourist trail.
Firstly, this edition was published in April 2009, and I traveled in August 2009 and at least 1/3 of the restaurants they recommend are no longer there! So... either Portugal has a big turn over of restaurants, or more likely they haven't been checked for this edition. For example, in Porto we looked for a restaurant recommended on the water front. After eventually finding it we discovered that all that was left was a shell which could not possibly have housed a functioning restaurant for at least a year, possibly longer! In Lisbon, a recommended pizza restaurant is now a bar - and has been for 2 years because we asked the barman! When we did find a recommendation that still existed, we wondered why it had been recommended because they were not very good. We often found better restaurants/hotels just looking further up the street, and regularly did this because we quickly lost faith in the book!
As another example, they recommend a "lush oasis" of a tropical botanic garden in Belem. Well, maybe it was a lush oasis when the first edition was written back in the early ninties, but it is now a rundown, empty dive of a place with no one but crickets!
On a positive note, they get most of the prices right for things like train times and museum entry. And their history section is useful for something to read while on a train.
But, there are no colour maps at the back (I thought this was a LP standard?) and no maps of the public transport systems of Lisbon or Porto. The black and white maps of Lisbon that are present overlap and don't fit particularly well with the individual areas that you might visit in one go. And, their recommendations for beaches and walks are often in places that are extremely difficult to get to by public transport.
Overall, I would recommend a different travel guide if you are going to Portugal, even if you are normally a loyal LP devotee like me!