My daughter is 10.5 months old. We have been using the Dr. Brown's bottles since she was born, and I have long ago gotten tired of washing all the parts. Since her 6 month well child visit, when the pediatrician mentioned it, I have sporadically tried to get her to use a sippy cup. I have one of the hard spout with valve Playtex cups, and I bought a Nuby one thinking it would be closer in shape and feel (the nipple) to the bottle nipple. Well, she just bites the Playtex spout and frequently the Nuby one as well; sometimes when very, very hungry she will drink maybe 2-3 oz of formula from the Nuby then cry until she gets a bottle. I somehow ran across the Thinkbaby trainer cup and thought, why not? The nipple seemed somewhat closer to a bottle nipple. Well, we received it 3 days ago and it has worked out great - the first time I tried it one morning, I put just 3 oz in it thinking I might have to finish up with the bottle. I ended up having to pour the rest of the formula in as she was doing so well - and since then I haven't used a bottle at all! She has even been drinking water from it which I hadn't been able to get her to do much. Regarding some of the complaints on here - perhaps my daughter is not as strong a sucker, but the nipple doesn't really seem to collapse much on her, or at least not even to affect the flow. It sometimes leaks some if you are shaking it up and down (e.g., to mix up the formula) but when she was rolling it around and upside tonight on her highchair tray, nothing spilled out. I am very excited to finally be able to put away the Dr. Brown bottles, all their many parts, and the bottle rack taking up space on our kitchen counter! Haven't traveled with it yet, and it will be harder to fit it into the insulated bottle pocket on our diaper backpack as they are fatter, but overall this is an awesome cup!! Oh, and SO EASY to clean! I'm buying at least one more.