At the very least, its fun to hear their tales. A good guidebook is important, but not a travel bible. A recent newspaper article I read stated that the right guidebook can make or break your trip. You definately need one, but if you get too tied to the book, you'll miss out on great towns, sights and hostels that aren't in any guidebook. After I travelled for a while, I stopped consulting my guidebook for anything more than the phone numbers and addresses of hostels.
Guidebooks vary in style, format, content and target audience.
Before you purchase a guidebook, flip through them and see which one appeals to you. Find the one that meshes best with your personality and what you want to get out of the trip. For what its worth, which admittedly is based on my own personal, biased opinion and experiences, I've included a quick review of a couple european guidebooks. I took Let's Go Europe and the Lonely Planet Western Europe with me on my trip. I had both because my cousin swore by the Lonely Planet and I got the Let's Go free.