I had really never thought much about sippy cups vs open cups. It wasn't until I read this thread that I gave it much thought. My strategy so far has been to start testing the open cups around 18 months, and if it doesn't go well then they go back to sippy cups. I currently only have 1 year olds and 2 year olds, and all the 2 year olds drink from open cups. They use their sippy cups when we go out in the wagon, and I give them cups with lids and straws when we have smoothies.