I have used these Hammer Nutrition gel flasks for my recent races and they are invaluable for numerous reasons. They are very easy to store in a pocket or down the front of your jersey for easy access. They are well marked for quantity - just keep in mind not all gels are the same volume. The two things I like most about these flasks the finger grips (easy to wrap your hand around - especially if you need to hold onto it while running) and the flow through the spout. Depending on gel viscosity, you can get a full dose of gel out in one squeeze - a major bonus for someone with any concerns about race-day fueling.
I am a competitive mountain bike racer and using these flasks is the fastest way to take on nutrition in the form of gels and keep moving forward. When I don't need to take a full 5 servings with me (meaning a completely full flask of gel) I always add water to thin out the mix. This not only makes the fueling time even quicker but also ensures you get all the gel out a lot easier. I have a buddy that adds a little brewed coffee to his flasks to add some caffeine.