Both Nicolas and Zachary have learned to drink out of a sippy cup all by themselves. Nicolas is great at handling the cup alone and drinks very well out of it. Zachary can drink out of it, but prefers to chew on the mouthpiece instead. He only drinks out of it when he is really thirsty.
It took us a few weeks of trying them with the sippy cups before they really got the hang of it. It made for a lot of meals with water all over themselves, their high chairs and the floor. At first, I was bringing the sippy cups to their mouths and giving them the water by holding the cup myself. Nicolas did not really care for that. He wanted the cup! Zachary was great and drank very well out of it that way. Then I would let them to "play" with the cups so that they could learn what they needed to about how the cups worked. But, once I knew that they were only playing with them instead of trying to figure them out, I stopped them from getting water all over the place.
After that point Nicolas was a pro. Zachary likes to chew on the rubbery mouthpiece and drinks occasionally. I figure that as long as he is not making a mess, it's fine. I think it feels great on his poor little gums. The good think is that when they have them, they know they are supposed to be drinking out of them. All I need to do is remind them and the cups end up back upright and in their mouths. They are really starting to understand when we speak. It's so amazing to watch them learn and discover the world...
Here are a few pics of my big boys and their sippy cups:
Zachary:

Nicolas:
