We just started using the avent sippy cups with our 6 month old. He was bottled fed from day one, but has adjusted to the sippy cup wonderfully. As far as the leaking issues, there has only been one time when it did leak, but we didn't have the lid screwed on correctly. Other than that one time, it hasn't leaked since. We highly recommend the avent bottles, bottle warmer, pacifiers, and the NO Leak sippy cups.