I tried a GPS a few years ago and was very disappointed. It was heavy, ate batteries, I never needed on a trail, and rarely worked under tree cover, but I know they have gotten better (but not lighter!). If you get one make sure to get one of the newer "high sensitivity" receivers or you will never get signal under tree cover or in a river gorge. I believe these are desinated by a "x" in the name of Garmin recievers. I have a backpacking buddy that has a Garmin Colorado (I am not sure which one) with the new high sinsitivity reciever and he has gotten satalites in Linville Gorge which probably means you can get signal about anywhere.
For day trips (where you don't have to worry about battaries or weight) I can see where one would be very benificial for marking spots that you wanted to go back and fish later (sometimes, I couldn't find a spot again I wanted to fish) especially if some time has passed between trips.