I just returned from a month-long trip to Argentina in July 2010. The newest Lonely Planet Argentina isn't coming out until September 2010 so we travelled with the older version. Although the guide has a warning about price increases, I was shocked to find almost every price quoted in the book to be about 40-50% higher than written. I do understand that prices in Argentina have skyrocketed in the past 10 years, but perhaps Lonely Planet should have researched a new edition sooner. The huge increase in prices almost doubled the budgeted cost of our trip.
That being said, I do feel the place and attraction descriptions were accurate and it was a good source of basic information from which to start researching where we wanted to go. The maps and addresses were accurate.
It has a HUGE Buenos Aires section, but we didn't spend much time there during the month so I can't honestly evaluate that section. We did have significant difficultly making reservations in Buenos Aires because at least 5 of the places listed in the book were full.
This guidebook also has a significant, more accurate, and well-written section on Uruguay. Although I am not sure why they didn't name it "Argentina and Uruguay" since Uruguay is another country, not a region of Argentina. My friends in Uruguay were quite insulted by this and felt it was "typical arrogant Argentine behavior". The information in the Uruguay section was more accurate that the info in the Argentina section.
Lastly, I felt the lodging and attractions listings were mostly geared toward travellers ages 18-25. We are a family travelling with two children, and often felt it hard to find affordable places to stay that weren't party hostels among the listings in the book. We even made reservations and paid a deposit at a place in Iguazu and found out once we arrived that they don't allow children! It would have been nice to read that ahead of time in the book. And I felt that there were very few activities and things to see that were geared toward families. Surely they exist!