I personally prefer to use a good sports drink (Accelerade, etc.), but I will also bring a few GU Gels with me if the ride will be 2 hours or longer. To me, one GU gel is more than adequate as long as I have a sports drink.
The one place I noticed that GU Gels really make a different is on long rides (75+ miles) where I'm in the saddle for more than 4 or 5 hours. In those situations though, I also use a sports drink and sports bars, but I try to take on at least one GU Gel every hour or hour and a half.
Like several people have mentioned, you won't feel them. But if you've ever bonked, you know why you need them.
Another important point... for long rides it's so very important to properly eat well before the ride (3 hours before). Grabbing a powerbar 10 minutes before a 6 hour ride when you haven't eaten anything else all day is not the smartest thing to do.