I used Rough Guides for my Italy trip last May, the guide to Tuscany and the guide to Florence & Siena. Very informative, gave lots of good historical background and cultural information. I also looked at the Lonely Planet and Rick Steves guides, but only the Rough Guide made it into my panniers. I intend to pick up the appropriate Rough Guides for Sicily, Sardinia, and Corsica for my May '08 trip.
Go to Amazon.com and read the customer reviews for all the different guide books for an area that you plan on visiting, then use the star ratings and what they actually say in the reviews to help you pick a guide. Different people have different tastes and requirements when they choose a guide book, so one person's "won't buy anyting else" could well be another person's "wouldn't touch it with a 10 foot pole".