I try to read up as much as possible before I go and I usually take one guidebook with me. I must agree with Rick Steves on this point, a guidebook is a $15 investment to make sure you get the most out of your $1500 trip (or words to that effect). I realize guidebooks are just the perception of the author, but it would be awful to find out after I returned home that I was within 20 miles or so of something that REALLY interested me and I didn't know about it.
I keep all my guidebooks, no matter how old they are. Things get dropped out of newer guidebooks sometimes. Otherwise, I'd participate in the gb exchange. It's a great idea.