There's a local low key "prediction" 5k I've run a few times. I always smuggle a watch in my back pocket so I can review my splits later, but don't look at it during the race. I'm not great at predicting (I don't run a lot of 5k's) but I do seem pretty good at running very consistent splits without a watch. My most recent I ran 5:51, 5:45, 5:47 and 5:50 for the final 0.16 (gps). It's always a little maddening during the race wondering if I'm going fast enough or way too fast.
For a marathon I feel like having the GPS is valuable to help me stay level headed early, and for the learning for the future. I like to try to learn from my races to see where I did well and where I blew up.