My stepfather, who is an avid cyclist, introduced me to Carb Gel recently. I had heard about them before, but didn't really bother to learn what they were for. During a visit to Phoenix, he sent me home with a bag full of GU and Hammer Gel also some Honey Stingers. Of the 3, the GU and Hammer Gel sat best with my stomach, the Honey Stinger on an empty stomach was not great. The other 2 seemed to be fine in that regard. It may be because the primary ingredient in the Honey stinger is honey which is different than the other 2. For this reason, I'm gonna talk about these 2:
These little pouches contain around 100 calories of carbs. Most of the carbs in both products come the complex carb maltodexterin which is supposed to not give you a sugar rush and crash. Some flavors of GU contain caffeine and some don't the ones that do contain 20mg and the Jet Blackberry GU has 40mg caffeine, the Espresso and Tropical flavors or Hammer Gel contain 25mg.
So far this has been my experience with them: I've put in several 9+ mile runs using these and have been very happy with my energy level. Both products recommend that you take 1 serving 15 minutes before your workout and one every 45 minutes during. At $1 to $1.50 a pouch, that could get expensive. You can mix it with some water and put it in a nutrition flask and spread it out more, that is what I've been doing and it works great. The Hammer gel also comes in 26 serving bottles for a lot less per serving, but then you have to mix it in another container.
Overall, if you are doing endurance sports like running, biking, etc. and find that you run out of juice, you might want to take a look at these gels as a quick source of energy that isn't going to sit bulky in the pit of your stomach for the next 5 miles.