I tried GU for the first time while about 2/3's of the way through the Boston Marathon. I nearly gagged on it, but in fairness, I would have found it hard to stomach anything at that point. By pure coincidence I came into possession of the scanned GU (the same flavor as I had in the marathon) as part of a goodie bag given to participants of the Groton Road Race.
I'm not sure what value the GU is--I tried it because...well, it was there and free. It only has 100 calories and I burn around 800/hour while racing, so I don't see how it could have much impact. Even if you could choke one down every 30 minutes that would only provide 1/4 of the calories being used.
I put in the Gulden's mustard packet for scale and because I love their graphic design.