I use Endurolytes whenever I go on very long rides and whenever it's hot enough that I worry about excessive sweating throwing off my electrolyte balance. They're great because they're easy to take (just swallow the caplets or, if you prefer, open the caplets and add them to your drink) and pretty easy to carry with you during exercise. You just take a couple of caplets every hour or two, depending on conditions. You can even get a little caplet dispenser (which makes your life much easier if you're doing extended training or racing) from Hammer Nutrition if you want.
Anecdotally, I had terrible problems with cramping -- especially in road races over 2 hours on hot days -- before I started using these. Now, I get cramps only very rarely. So they've really helped me.

Ditto what everyone else said about cramps. I wish I had know about these much earlier in life although I am not sure this product has existed for very long. I have had serious problems with cramps and muscle spasms while involved in any kind of lenthy endurance activity. This problem has been eliminated and I feel better overall during such events including backpacking and cycling.

Iv'e used endurolytes for every Ironman I have done. Without these I would have never been able to do as well as I have in all my races. They work great with your sports drink and really prevent the cramping and muscle fatique that accompanies endurance training. I have actually had people come up to me during races and ask if I have any extra endurolyte's. A must have for any athlete