I was using water alone, but on rides longer than 3 hours water can lead to problems, see: http://chemistry.about.com/cs/5/f/blwaterintox.htm
First, water isn't as well absorbed as water with electrolytes, so if you have water by itself, it can just sit in your stomach, and if you get enough of it, it can cause GI distress.
Second, a rider can be sweating out up to a quart of water a hour. Also, you can lose anywhere from 500mg to 1000mg of salt per liter of sweat, so it's easy to get down on salt.
So, drinking limited amounts of water can lead to both dehydrated and hyponatremic.
In hot weather, I take two salt capsules per hour. I use succeed! e-caps. I don't like the endurolytes - hammer has a philosophy that salt isn't important and you can only get 240mg/hour from them (I get more than that from 1 of the capsules I take).
Things to look for:
If you weigh more after the ride than before, you are likely hyponatremic. If you drink lots of water and make no trips to the bathroom, you are likely hyponatremic. If your hands look swolen, you are likely hyponatremic.
Hyponatremia is nothing to sneeze at. The symptoms are similar to dehydration and heatstroke and many medical personel are not trained to deal with people who exercise for more than 5 hours at a time.