Lonely Planet has broken down Argentina into nine geographical areas and the guide is at its best outside of Buenos Aires. The guide excels at providing you with insight into the culture, history and uniqueness of the wonderful countryside of Argentina. But, in Buenos Aires it falls short.
In three years much has changed in Buenos Aires' hotel prices. Though Lonely Planet has a published date of 2005, the prices the guide quotes for accommodations go back to 2003/2004. For example, the Howard Johnson's Hotel in Microcentro, is quoted in the guide as $51 a night; now (3-2006) the price now is $113. The Hotel NH Jousten is quoted at $129, now it is $210. Everything, except for the dorm rooms at youth hostels, has doubled, or even tripled, in price..
Also, Buenos Aires is a place to die for when it comes to great restaurants. Sadly, this guides misses many of the best. For example, of the thirty or more restaurants on the Puerto Madero waterfront, Lonely Planet lists only three, and two of these were sub-par. Also, though restaurants prices have not tripled in three years, they are about 30 to 40 % higher than quoted in the guide. The guide does not give what time the restaurants open; so take care, especially on weekends, or you can show up and find the doors locked.
Maps, and their quality, are very important. A map should tell you where to eat, sleep, what to visit, and do so quickly. Lonely Planet makes this difficult; these maps are cumbersome to use and hard to read.
Where this guide shines is its coverage of the rest of Argentina. The recommended accommodations and restaurants are very good and the prices quoted are reliable. Lonely Planet is one of the few guides that will give you the population and altitude (in meters) of major cities and towns. Kudos! You will find the descriptions of the towns and cites are top rate, and of course, all the "must see" sights are listed and explained. Each region has a good historical sketch and many great tips.
This guide goes head-to-head with Rough Guide, and between the two, I would take Rough Guide. However, if you are skipping Buenos Aires and going to explore the countryside, then consider Lonely Planet. Best yet, take both guides.